demux_mkv: use a more universal zero initializer

Unfortunately, we can't avoid this warning 100%, because ebml_info is
written by a Perl script. I think the script writes the struct fields in
random order (thanks Perl), so there's no way to know whether the first
struct field is a scalar or a struct.

At least {0} is always valid here, even if it shows a warning. (The
compilers are wrong, see e.g. [1].)

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

demux/demux_mkv.c

@@ -359,7 +359,7 @@ static int demux_mkv_read_info(demuxer_t *demuxer)
     mkv_d->tc_scale = 1000000;
     mkv_d->duration = 0;
 
-    struct ebml_info info = {{0}};
+    struct ebml_info info = {0};
     struct ebml_parse_ctx parse_ctx = {0};
     if (ebml_read_element(s, &parse_ctx, &info, &ebml_info_desc) < 0)
         return -1;