sys/: Explicitly create our custom buffer classes at a thread-safe location as well, since g_type_class_ref() doesn't...

Original commit message from CVS:
* sys/ximage/ximagesink.c: (gst_ximagesink_get_type):
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_get_type):
Explicitly create our custom buffer classes at a thread-safe
location as well, since g_type_class_ref() doesn't seem to be
entirely thread-safe either (#365501; also see #349410).