Change 'arg' type in parse_meta_type, fix warning:

ffmpeg.c: In function ‘parse_meta_type’:
ffmpeg.c:3323:13: warning: assignment discards qualifiers from pointer target type

Originally committed as revision 26312 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -2984,7 +2984,7 @@ static void opt_map(const char *arg)
     }
 }
 
-static void parse_meta_type(const char *arg, char *type, int *index, char **endptr)
+static void parse_meta_type(char *arg, char *type, int *index, char **endptr)
 {
     *endptr = arg;
     if (*arg == ',') {