Depend on GLib 2.12 and use it unconditionally as we do in other modules too already.

Depend on GLib 2.12 and use it unconditionally as we do in other modules too already.

Original commit message from CVS:
* configure.ac:
* gst/mpegaudioparse/gstmpegaudioparse.c:
(mpeg_audio_seek_entry_free):
* gst/mpegaudioparse/gstxingmux.c: (gst_xing_seek_entry_free):
Depend on GLib 2.12 and use it unconditionally as we do in other
modules too already.