gst: don't use volatile to mean atomic

volatile is not sufficient to provide atomic guarantees and real atomics
should be used instead.  GCC 11 has started warning about using volatile
with atomic operations.

https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719

Discovered in gst-plugins-good#868

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1219>

gst-libs/gst/gl/egl/gstglmemoryegl.c

@@ -266,7 +266,7 @@ gst_gl_memory_egl_allocator_init (GstGLMemoryEGLAllocator * allocator)
 void
 gst_gl_memory_egl_init_once (void)
 {
-  static volatile gsize _init = 0;
+  static gsize _init = 0;
 
   if (g_once_init_enter (&_init)) {
     gst_gl_memory_init_once ();