windows: append executable extension (done as per Russ' suggestion)

R=rsc
CC=brainman, golang-dev
https://golang.org/cl/1690053
diff --git a/src/Make.cmd b/src/Make.cmd
index b2a184b..b2b764f 100644
--- a/src/Make.cmd
+++ b/src/Make.cmd
@@ -11,6 +11,10 @@
 
 include $(QUOTED_GOROOT)/src/Make.common
 
+ifeq ($(GOOS),windows) 
+TARG:=$(TARG).exe 
+endif
+
 PREREQ+=$(patsubst %,%.make,$(DEPS))
 
 $(TARG): _go_.$O $(OFILES)