Missing const

Originally committed as revision 12147 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ratecontrol.c

@@ -66,7 +66,7 @@ int ff_rate_control_init(MpegEncContext *s)
 {
     RateControlContext *rcc= &s->rc_context;
     int i;
-    char *error = NULL;
+    const char *error = NULL;
     static const char *const_names[]={
         "PI",
         "E",