ffplay: add cast to fix warning in configure_video_filters()

Fix warning:
ffplay.c:1578:40: warning: passing argument 5 of ‘avfilter_graph_create_filter’ discards ‘const’ qualifier from pointer target type [enabled by default]
libavfilter/avfiltergraph.h:84:5: note: expected ‘void *’ but argument is of type ‘const enum PixelFormat *’

ffplay.c

@@ -1575,7 +1575,7 @@ static int configure_video_filters(AVFilterGraph *graph, VideoState *is, const c
 #if FF_API_OLD_VSINK_API
     ret = avfilter_graph_create_filter(&filt_out,
                                        avfilter_get_by_name("buffersink"),
-                                       "out", NULL, pix_fmts, graph);
+                                       "out", NULL, (void *)pix_fmts, graph);
 #else
     buffersink_params->pixel_fmts = pix_fmts;
     ret = avfilter_graph_create_filter(&filt_out,