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

Since 97965478

 ("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>
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