Reduce the number of exported symbols from the native lookup library

[andrisaar]

This ensures that the .so only has two global symbols that it exports: oak_main and register_globals, both following the C calling convention. These are the only symbols that we expect external code to call.

Without this change we have 811 exported symbols:

$ nm bazel-bin/cc/native_sdk/libkey_value_lookup.so | grep " T " | wc -l
811

After that, it's down to the two we'd expect:

$ nm bazel-bin/cc/native_sdk/libkey_value_lookup.so | grep " T "
0000000000009f1f T oak_main
000000000000a9e6 T register_callbacks