common.mk: ignore clean-rubyspec error [ci skip]

* common.mk (clean-rubyspec): nothing to clean before running
  rubyspec.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57827 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -620,7 +620,7 @@ clean-platform:
 
 RUBYSPEC_CAPIEXT = spec/rubyspec/optional/capi/ext
 clean-rubyspec: PHONY
-	$(Q) $(RM) $(RUBYSPEC_CAPIEXT)/*.$(OBJEXT) $(RUBYSPEC_CAPIEXT)/*.$(DLEXT)
+	-$(Q) $(RM) $(RUBYSPEC_CAPIEXT)/*.$(OBJEXT) $(RUBYSPEC_CAPIEXT)/*.$(DLEXT)
 	-$(Q) $(RMDIRS) $(RUBYSPEC_CAPIEXT)
 
 check: main test test-testframework test-almost