edje_cc - fix warning about freeing a const char ptr

yes - the type is const char * for edje runtime (the library) but in
edje_cc we strdup these strings and yes - assign them to a const char *
thus the warning and the need to cast. it's specific co edje_cc only
and so this is correct.

src/bin/edje/edje_cc_out.c

@@ -1419,7 +1419,7 @@ data_write_images(Eet_File *ef, int *image_num)
                   char *tmp = malloc(size);
                   snprintf(tmp, size, "%s.png", img->entry);
                   INF("Vector '%s' used as image, convert to bitmap '%s'", img->entry, tmp);
-                  free(img->entry);
+                  free((void *)img->entry);
                   img->entry = tmp;
                }
           }