build: fix rust parallel build race.

plugins/Makefile has target/${RUST_PROFILE}/cln-grpc depend on the
generated files via $(MSGGEN_GENALL), but cln-rpc/Makefile adds to
that variable, so needs to be included first.

Here's an example build error:

```
Combining schemas from /home/rusty/lightning-ltest/doc/schemas into /home/rusty/lightning-ltest/contrib/msggen/msggen/schema.json
Created /home/rusty/lightning-ltest/contrib/msggen/msggen/schema.json from 2 files
error: failed to run custom build command for `cln-grpc v0.3.0 (/home/rusty/lightning-ltest/cln-grpc)`

Caused by:
  process didn't exit successfully: `/home/rusty/lightning-ltest/target/debug/build/cln-grpc-95489e3ba33c0ab3/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=proto/node.proto
  cargo:rerun-if-changed=proto

  --- stderr
  thread 'main' panicked at cln-grpc/build.rs:7:10:
  called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "protoc failed: node.proto:134:52: \"AskreneageResponse\" is not defined.\nnode.proto:135:23: \"GetroutesRequest\" is not defined.\nnode.proto:135:50: \"GetroutesResponse\" is not defined.\nnode.proto:136:32: \"AskrenedisablenodeRequest\" is not defined.\nnode.proto:136:68: \"AskrenedisablenodeResponse\" is not defined.\nnode.proto:137:34: \"AskreneinformchannelRequest\" is not defined.\nnode.proto:137:72: \"AskreneinformchannelResponse\" is not defined.\nnode.proto:138:34: \"AskrenecreatechannelRequest\" is not defined.\nnode.proto:138:72: \"AskrenecreatechannelResponse\" is not defined.\nnode.proto:139:34: \"AskreneupdatechannelRequest\" is not defined.\nnode.proto:139:72: \"AskreneupdatechannelResponse\" is not defined.\nnode.proto:140:32: \"AskrenebiaschannelRequest\" is not defined.\nnode.proto:140:68: \"AskrenebiaschannelResponse\" is not defined.\nnode.proto:141:37: \"AskrenelistreservationsRequest\" is not defined.\nnode.proto:141:78: \"AskrenelistreservationsResponse\" is not defined.\nnode.proto:142:32: \"InjectpaymentonionRequest\" is not defined.\nnode.proto:142:68: \"InjectpaymentonionResponse\" is not defined.\nnode.proto:143:18: \"XpayRequest\" is not defined.\nnode.proto:143:40: \"XpayResponse\" is not defined.\nnode.proto:145:33: \"StreamBlockAddedRequest\" is not defined.\nnode.proto:145:74: \"BlockAddedNotification\" is not defined.\nnode.proto:146:40: \"StreamChannelOpenFailedRequest\" is not defined.\nnode.proto:146:88: \"ChannelOpenFailedNotification\" is not defined.\nnode.proto:147:36: \"StreamChannelOpenedRequest\" is not defined.\nnode.proto:147:80: \"ChannelOpenedNotification\" is not defined.\nnode.proto:148:30: \"StreamConnectRequest\" is not defined.\nnode.proto:148:68: \"PeerConnectNotification\" is not defined.\nnode.proto:149:32: \"StreamCustomMsgRequest\" is not defined.\nnode.proto:149:72: \"CustomMsgNotification\" is not defined.\n" }
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
make: *** [plugins/Makefile:305: target/debug/cln-grpc] Error 101
make: *** Waiting for unfinished jobs....
```

Signed-off-by: Rusty Russell <[email protected]>

Makefile

@@ -369,17 +369,15 @@ include doc/Makefile
 include contrib/msggen/Makefile
 include devtools/Makefile
 include tools/Makefile
+ifneq ($(RUST),0)
+include cln-rpc/Makefile
+endif
 include plugins/Makefile
 include tests/plugins/Makefile
 
 ifneq ($(FUZZING),0)
 	include tests/fuzz/Makefile
 endif
-ifneq ($(RUST),0)
-	include cln-rpc/Makefile
-	include plugins/rest-plugin/Makefile
-endif
-include cln-grpc/Makefile
 
 ifneq ($V,1)
 MSGGEN_ARGS := -s

plugins/Makefile

@@ -307,6 +307,7 @@ target/${RUST_PROFILE}/clnrest: ${CLN_REST_PLUGIN_SRC}
 	cargo build ${CARGO_OPTS} --bin clnrest
 
 ifneq ($(RUST),0)
+include plugins/rest-plugin/Makefile
 DEFAULT_TARGETS += $(CLN_PLUGIN_EXAMPLES) plugins/cln-grpc plugins/clnrest
 endif