gdbus-peer: Make sure to not include objectmanager-gen.c source

The executable depends only on the generated header file at compile time, and on the library at link time. So meson can decide to compile gdbus-peer.c before compiling the library and thus won't have generated the header yet, causing the build error.

So declare_dependency() should only have the header file in its sources, to force generating files before compiling gdbus-peer.c without including objectmanager-gen.c into gdbus-peer's sources.