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.