automoc: silence linker warning on macos

when an empty `mocs_compilation.cpp` is included in a static library,
the macos linker emits a warning:
```
/Applications/Xcode13.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib:
file: path/To/Library.a(mocs_compilation.cpp.o) has no symbols
```

we therefore generate a globally visible weak symbol in the form of a
virtual function table
38 jobs for !7296 with feature/silence-ranlib-warning in 61 minutes and 14 seconds (queued for 3 seconds)
merge request