Add missing dependency

just fix CI failure
https://travis-ci.org/ruby/ruby/jobs/569625233

common.mk

@@ -1609,6 +1609,7 @@ 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