refs.h: add forward declaration for structs used in this file

Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs.h

 #ifndef REFS_H
 #define REFS_H
 
+struct object_id;
+struct strbuf;
+struct string_list;
+
 /*
  * Resolve a reference, recursively following symbolic refererences.
  *