Makefile.am: Fix race when calling gdbus-codegen

gdbus-codegen generates multiple files, which is annoying. This causes
make to call it more than once at the same time, when doing parallel
jobs.

Force make to call gdbus-codegen for only the generated .c files, and
expect that the corresponding .h file will be generated in the process.

Closes #274

Reviewed-by: Stephen Gallagher <sgallagh@redhat.com>