kbuild: document 'make prepare' in 'make help'

The output of 'make help' covers a lot of options, but doesn't include
a listing for 'make prepare'.  Here's a one-liner to fix that...
Signed-off-by: NValdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1181,6 +1181,7 @@ help:
 	@echo  '  dir/            - Build all files in dir and below'
 	@echo  '  dir/file.[ois]  - Build specified target only'
 	@echo  '  dir/file.ko     - Build module including final link'
+	@echo  '  prepare         - Set up for building external modules'
 	@echo  '  tags/TAGS	  - Generate tags file for editors'
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  kernelrelease	  - Output the release version string'