gst/gstinfo.*: Add some G_[UN]LIKELY.

gst/gstinfo.*: Add some G_[UN]LIKELY.

Original commit message from CVS:
* gst/gstinfo.c: (gst_debug_set_active),
(gst_debug_category_set_threshold), (_gst_debug_nameof_funcptr):
* gst/gstinfo.h:
Add some G_[UN]LIKELY.
Maintain __gst_debug_min to avoid formatting the arguments of
debug messages that will be dropped anyway to avoid a lot of
overhead from the debugging system.