Add const qualifier to the error var, fix warnings.

Originally committed as revision 17435 to svn://svn.ffmpeg.org/ffmpeg/trunk

vhook/imlib2.c

@@ -151,7 +151,7 @@ int Configure(void **ctxp, int argc, char *argv[])
     char *color = 0;
     FILE *f;
     char *p;
-    char *error;
+    const char *error;
 
     *ctxp = av_mallocz(sizeof(ContextInfo));
     ci = (ContextInfo *) *ctxp;