Fix highmem PPC build failure

Commit f4112de6 ("mm: introduce
debug_kmap_atomic") broke PPC builds with CONFIG_HIGHMEM=y:

   CC      init/main.o
  In file included from include/linux/highmem.h:25,
                   from include/linux/pagemap.h:11,
                   from include/linux/mempolicy.h:63,
                   from init/main.c:53:
  arch/powerpc/include/asm/highmem.h: In function 'kmap_atomic_prot':
  arch/powerpc/include/asm/highmem.h:98: error: implicit declaration of function 'debug_kmap_atomic'
  In file included from include/linux/pagemap.h:11,
                   from include/linux/mempolicy.h:63,
                   from init/main.c:53:
  include/linux/highmem.h: At top level:
  include/linux/highmem.h:196: warning: conflicting types for 'debug_kmap_atomic'
  include/linux/highmem.h:196: error: static declaration of 'debug_kmap_atomic' follows non-static declaration
  include/asm/highmem.h:98: error: previous implicit declaration of 'debug_kmap_atomic' was here
  make[1]: *** [init/main.o] Error 1
  make: *** [init] Error 2

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>