libxvid: Add const qualifier to silence compiler warning.

libavcodec/libxvidff.c:752: warning: initialization discards qualifiers from pointer target type

libavcodec/libxvidff.c

@@ -749,7 +749,7 @@ static int xvid_ff_2pass_before(struct xvid_context *ref,
 static int xvid_ff_2pass_after(struct xvid_context *ref,
                                 xvid_plg_data_t *param) {
     char *log = ref->twopassbuffer;
-    char *frame_types = " ipbs";
+    const char *frame_types = " ipbs";
     char frame_type;
 
     /* Quick bounds check */