rtp: fix g-i warnings
Use same variable name in function declaration as in function definition and gtk-doc/g-i blurb.
gstrtpmeta.c:36: Warning: GstRtp: gst_buffer_add_rtp_source_meta: unknown parameter 'buffer' in documentation comment, should be 'buf'
gstrtpmeta.c:80: Warning: GstRtp: gst_buffer_get_rtp_source_meta: unknown parameter 'buffer' in documentation comment, should be 'buf'