kbuild: Add cscope to make help

From: Bjorn Helgas <bjorn.helgaas@hp.com>

Makefile

@@ -921,6 +921,7 @@ help:
 	@echo  '  dir/file.[ois]  - Build specified target only'
 	@echo  '  rpm		  - Build a kernel as an RPM package'
 	@echo  '  tags/TAGS	  - Generate tags file for editors'
+	@echo  '  cscope	  - Generate cscope index'
 	@echo  ''
 	@echo  'Documentation targets:'
 	@$(MAKE) -f $(srctree)/Documentation/DocBook/Makefile dochelp