i2pd: mark as broken on darwin

Currently fails with error:

```
libi2pd/FS.h:21:10: error: 'TARGET_OS_SIMULATOR' is not defined, evaluates to 0 [-Werror,-Wundef-prefix=TARGET_OS_]
               ^
1 error generated.
```

pkgs/tools/networking/i2pd/default.nix

@@ -44,5 +44,6 @@ stdenv.mkDerivation rec {
     maintainers = with maintainers; [ edwtjo ];
     platforms = platforms.unix;
     mainProgram = "i2pd";
+    broken = stdenv.hostPlatform.isDarwin;
   };
 }