Move back `mrb_define_alias` to `mruby.h` to avoid breakage.

include/mruby.h

@@ -1197,6 +1197,7 @@ typedef enum call_type {
   CALL_TYPE_MAX
 } call_type;
 
+MRB_API void mrb_define_alias(mrb_state *mrb, struct RClass *c, const char *a, const char *b);
 MRB_API const char *mrb_class_name(mrb_state *mrb, struct RClass* klass);
 MRB_API void mrb_define_global_const(mrb_state *mrb, const char *name, mrb_value val);
 

include/mruby/class.h

@@ -80,7 +80,6 @@ MRB_API struct RClass *mrb_vm_define_module(mrb_state*, mrb_value, mrb_sym);
 MRB_API void mrb_define_method_raw(mrb_state*, struct RClass*, mrb_sym, mrb_method_t);
 MRB_API void mrb_define_method_id(mrb_state *mrb, struct RClass *c, mrb_sym mid, mrb_func_t func, mrb_aspec aspec);
 MRB_API void mrb_alias_method(mrb_state*, struct RClass *c, mrb_sym a, mrb_sym b);
-MRB_API void mrb_define_alias(mrb_state*, struct RClass *c, const char* a, const char* b);
 
 MRB_API mrb_method_t mrb_method_search_vm(mrb_state*, struct RClass**, mrb_sym);
 MRB_API mrb_method_t mrb_method_search(mrb_state*, struct RClass*, mrb_sym);