ffplay: add braces around union definition

Fix warning:
ffplay.c: At top level:
ffplay.c:3027: warning: missing braces around initializer
ffplay.c:3027: warning: (near initialization for ‘options[53].u’)

ffplay.c

@@ -3024,7 +3024,7 @@ static const OptionDef options[] = {
 #endif
     { "rdftspeed", OPT_INT | HAS_ARG| OPT_AUDIO | OPT_EXPERT, {(void*)&rdftspeed}, "rdft speed", "msecs" },
     { "default", OPT_FUNC2 | HAS_ARG | OPT_AUDIO | OPT_VIDEO | OPT_EXPERT, {(void*)opt_default}, "generic catch all option", "" },
-    { "i", OPT_DUMMY, NULL, "ffmpeg compatibility dummy option", ""},
+    { "i", OPT_DUMMY, {NULL}, "ffmpeg compatibility dummy option", ""},
     { NULL, },
 };