Fixed the each object callback to be more consistent with other callbacks.

src/gc.c

@@ -1153,7 +1153,7 @@ mrb_objspace_each_objects(mrb_state *mrb, each_object_callback* callback, void *
         p = page->objects;
         pend = p + MRB_HEAP_PAGE_SIZE;
         for (;p < pend; p++) {
-           callback(mrb, &p->as.basic, data);
+           (*callback)(mrb, &p->as.basic, data);
         }
 
         page = page->next;