Fixed compile error when ADJUST_SKILL_DAMAGE macro is defined

src/map/map.h

@@ -590,7 +590,7 @@ struct s_skill_damage {
 		other; ///< Rate to Other target
 	uint8 caster; ///< Caster type
 };
-struct eri *map_skill_damage_ers;
+extern struct eri *map_skill_damage_ers;
 #endif
 
 struct questinfo_req {