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/Kbuild b/Kbuild
index 32f19c5..e750e9c 100644
--- a/Kbuild
+++ b/Kbuild
@@ -96,5 +96,4 @@
 	$(call cmd,syscalls)
 
 # Delete all targets during make clean
-clean-files := $(addprefix $(objtree)/,$(targets))
-
+clean-files := $(addprefix $(objtree)/,$(filter-out $(bounds-file) $(offsets-file),$(targets)))