opencore-amr: Add missing initializer braces to shut up gcc warning.

This fixes the warning:
libavcodec/libopencore-amr.c:91: warning: missing braces around initializer

libavcodec/libopencore-amr.c

@@ -88,7 +88,7 @@ typedef struct AMRContext {
 } AMRContext;
 
 static const AVOption options[] = {
-    { "dtx", "Allow DTX (generate comfort noise)", offsetof(AMRContext, enc_dtx), FF_OPT_TYPE_INT, 0, 0, 1, AV_OPT_FLAG_AUDIO_PARAM | AV_OPT_FLAG_ENCODING_PARAM },
+    { "dtx", "Allow DTX (generate comfort noise)", offsetof(AMRContext, enc_dtx), FF_OPT_TYPE_INT, { 0 }, 0, 1, AV_OPT_FLAG_AUDIO_PARAM | AV_OPT_FLAG_ENCODING_PARAM },
     { NULL }
 };