build: fixed a build cmp warning

mk/sketch_sources.mk

@@ -74,7 +74,7 @@ $(SKETCHCPP): showflags $(SKETCHCPP_SRC)
 	$(v)echo "#line 1 \"autogenerated\""                              >> $@.new
 	$(v)$(AWK)                '$(SKETCH_PROTOTYPER)' $(SKETCHCPP_SRC) >> $@.new
 	$(v)$(AWK) -v mode=body   '$(SKETCH_SPLITTER)'   $(SKETCHCPP_SRC) >> $@.new
-	$(v)cmp $@ $@.new 2>/dev/null || mv $@.new $@
+	$(v)cmp $@ $@.new > /dev/null 2>&1 || mv $@.new $@
 	$(v)rm -f $@.new
 
 # delete the sketch.cpp file if a processing error occurs