opt: Fix "libavutil/opt.c:811:20: warning: initialization discards ‘const’...

opt: Fix "libavutil/opt.c:811:20: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]"
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.c

@@ -808,7 +808,7 @@ const AVClass *av_opt_child_class_next(const AVClass *parent, const AVClass *pre
 
 void *av_opt_ptr(const AVClass *class, void *obj, const char *name)
 {
-    AVOption *opt= av_opt_find2(&class, name, NULL, 0, AV_OPT_SEARCH_FAKE_OBJ, NULL);
+    const AVOption *opt= av_opt_find2(&class, name, NULL, 0, AV_OPT_SEARCH_FAKE_OBJ, NULL);
     if(!opt)
         return NULL;
     return (uint8_t*)obj + opt->offset;