gst/gst.c: Make warning about late g_thread_init() calls a bit more explicit, so that it's more obvious to applicatio...

Original commit message from CVS:
* gst/gst.c: (gst_init_get_option_group):
Make warning about late g_thread_init() calls a bit more explicit,
so that it's more obvious to application developers what they need
to do if a user files a bug against their application.