Improve prototype for `mrb_objspace_page_slot_size()`; ref #5032

If it qualify a return type that is not a pointer with `const`, the
compiler ignores it.

include/mruby/gc.h

@@ -21,7 +21,7 @@ struct mrb_state;
 #define MRB_EACH_OBJ_BREAK 1
 typedef int (mrb_each_object_callback)(struct mrb_state *mrb, struct RBasic *obj, void *data);
 void mrb_objspace_each_objects(struct mrb_state *mrb, mrb_each_object_callback *callback, void *data);
-const mrb_int mrb_objspace_page_slot_size();
+mrb_int mrb_objspace_page_slot_size(void);
 MRB_API void mrb_free_context(struct mrb_state *mrb, struct mrb_context *c);
 
 #ifndef MRB_GC_ARENA_SIZE

src/gc.c

@@ -1599,8 +1599,8 @@ mrb_objspace_each_objects(mrb_state *mrb, mrb_each_object_callback *callback, vo
   }
 }
 
-const mrb_int
-mrb_objspace_page_slot_size()
+mrb_int
+mrb_objspace_page_slot_size(void)
 {
   const mrb_int i = sizeof(RVALUE);
   return i;