gst/: Make gtk-doc generate docs for our inlined gst_buffer_ref(), gst_event_ref() and gst_message_ref() functions ag...

Original commit message from CVS:
* gst/gstbuffer.h:
* gst/gstevent.h:
* gst/gstmessage.h:
Make gtk-doc generate docs for our inlined gst_buffer_ref(),
gst_event_ref() and gst_message_ref() functions again
(ugly hack, please do fix if there's a better way besides
overrides.txt, which doesn't seem to work).