autoconvert: fix compiler warnings with g_atomic on recent GLib versions

(Not sure why we're doing all this GAtomic stuff instead of just using simple locking)

The volatile is not needed here and causes compiler warnings with newer GLib versions.

gstautoconvert.c: In function ‘gst_auto_convert_dispose’ (and elsewhere): glib/gatomic.h:108:3: warning: initialization discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers] gstautoconvert.c:224:24: note: in expansion of macro ‘g_atomic_pointer_get’ 224 | GList *factories = g_atomic_pointer_get (&autoconvert->factories);