oggenc: add missing initializer braces to shut up gcc warning.

This fixes the warning:
    libavformat/oggenc.c:75:7: warning: missing braces around initializer [-Wmissing-braces]

libavformat/oggenc.c

@@ -72,7 +72,7 @@ typedef struct {
 
 static const AVOption options[] = {
     { "oggpagesize", "Set preferred Ogg page size.",
-      offsetof(OGGContext, pref_size), FF_OPT_TYPE_INT, 0, 0, MAX_PAGE_SIZE, AV_OPT_FLAG_ENCODING_PARAM},
+      offsetof(OGGContext, pref_size), FF_OPT_TYPE_INT, {.dbl = 0}, 0, MAX_PAGE_SIZE, AV_OPT_FLAG_ENCODING_PARAM},
     { NULL },
 };