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

include/mruby/gc.h

@@ -10,7 +10,7 @@
 #include "mruby.h"
 #include "mruby/value.h"
 
-typedef int each_object_callback(mrb_state *mrb, struct RBasic* obj, void *data);
+typedef void (each_object_callback)(mrb_state *mrb, struct RBasic* obj, void *data);
 void mrb_objspace_each_objects(mrb_state *mrb, each_object_callback* callback, void *data);
 
 #endif  /* MRUBY_GC_H */

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;