remove useless include and dependency

class.c

@@ -308,7 +308,7 @@ class_init_copy_check(VALUE clone, VALUE orig)
 	rb_raise(rb_eTypeError, "can't copy singleton class");
     }
 }
-#include "gc.h"
+
 /* :nodoc: */
 VALUE
 rb_mod_init_copy(VALUE clone, VALUE orig)

common.mk

@@ -1609,7 +1609,6 @@ class.$(OBJEXT): {$(VPATH)}config.h
 class.$(OBJEXT): {$(VPATH)}constant.h
 class.$(OBJEXT): {$(VPATH)}defines.h
 class.$(OBJEXT): {$(VPATH)}encoding.h
-class.$(OBJEXT): {$(VPATH)}gc.h
 class.$(OBJEXT): {$(VPATH)}id.h
 class.$(OBJEXT): {$(VPATH)}id_table.h
 class.$(OBJEXT): {$(VPATH)}intern.h