avcodec: Suppress deprecation warnings from avcodec_alloc_frame()

The function is itself obsolete and slated for removal.

libavcodec/utils.c

@@ -853,7 +853,9 @@ AVFrame *avcodec_alloc_frame(void)
     if (frame == NULL)
         return NULL;
 
+FF_DISABLE_DEPRECATION_WARNINGS
     avcodec_get_frame_defaults(frame);
+FF_ENABLE_DEPRECATION_WARNINGS
 
     return frame;
 }