common.mk: null device name

* common.mk (clean-platform): use $(NULL) as portabile null device
  name instead of hard coded /dev/null for the portability.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46987 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -509,7 +509,7 @@ clean-capi distclean-capi realclean-capi:
 
 clean-platform:
 	$(Q) $(RM) $(PLATFORM_D)
-	-$(Q) $(RMDIR) $(PLATFORM_DIR) 2> /dev/null || exit 0
+	-$(Q) $(RMDIR) $(PLATFORM_DIR) 2> $(NULL) || exit 0
 
 check: main test test-all
 	$(ECHO) check succeeded