gdbus-codegen: Fix header include in the body file

When body file is generated, the header name to be included is
built by using the path passed by `--output` directory. However,
this might not be correct because, if the path includes a
subdirectory, the whole path will be used instead of only the
base name.

Please see:
  https://github.com/mesonbuild/meson/issues/3488
  https://github.com/gnome-mpv/gnome-mpv/issues/305#issuecomment-385056156

https://bugzilla.gnome.org/show_bug.cgi?id=795802