Fixed missing forward declaration of mail_message.

git-svn-id: https://svn.code.sf.net/p/rathena/svn/trunk@11592 54d463be-8e91-2dee-dedb-b68131a5f0ec

src/map/intif.h

@@ -10,6 +10,7 @@ struct guild_member;
 struct guild_position;
 struct s_pet;
 struct s_homunculus;
+struct mail_message;
 
 int intif_parse(int fd);