fix ksm.h breakage of nommu build

Commit 5ad64688

 "ksm: let shared pages
be swappable" breaks the build on m68knommu and I suspect on any nommu:

  In file included from kernel/fork.c:52:
  include/linux/ksm.h:129: warning: 'enum ttu_flags' declared inside parameter list
  include/linux/ksm.h:129: warning: its scope is only this definition or declaration, which is probably not what you want
  include/linux/ksm.h:129: error: parameter 2 ('flags') has incomplete type
  make[1]: *** [kernel/fork.o] Error 1
  make[1]: *** Waiting for unfinished jobs....

Let's fix that with CONFIG_MMU around most of the !CONFIG_KSM declarations.

Reported-by: Steven King <sfking@fdwdc.com>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Tested-by: Steven King <sfking@fdwdc.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>