Don't clean bounds.h and asm-offsets.h

Since 97965478a66fbdf0f4ad5e4ecc4828f0cb548a45 ("mm: Get rid of __ZONE_COUNT")
mmzone.h includes bounds.h.
Calling make clean after make prepare removes bounds.h
again so when building external modules this fails.

Signed-off-by: Jan Blunck <jblunck@suse.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
--
diff --git a/Makefile b/Makefile
index d4aa4378..f18631f 100644
--- a/Makefile
+++ b/Makefile
@@ -1114,6 +1114,7 @@
 MRPROPER_FILES += .config .config.old include/asm .version .old_version \
                   include/linux/autoconf.h include/linux/version.h      \
                   include/linux/utsrelease.h                            \
+                  include/linux/bounds.h include/asm*/asm-offsets.h     \
 		  Module.symvers tags TAGS cscope*
 
 # clean - Delete most, but leave enough to build external modules