staging: lustre: include linux/highmem.h when needed

Something in recent linux-next kernels caused linux/highmem.h to
no longer be included implicitly from o2iblnd_cb.c, causing a build
failure:

drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd_cb.c: In function 'kiblnd_kvaddr_to_page':
drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd_cb.c:549:15: error: 'PKMAP_BASE' undeclared (first use in this function); did you mean 'RTM_BASE'?
  if (vaddr >= PKMAP_BASE &&
               ^~~~~~~~~~
               RTM_BASE
drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd_cb.c:549:15: note: each undeclared identifier is reported only once for each function it appears in
drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd_cb.c:550:28: error: 'LAST_PKMAP' undeclared (first use in this function); did you mean 'AT_HWCAP'?
      vaddr < (PKMAP_BASE + LAST_PKMAP * PAGE_SIZE)) {
                            ^~~~~~~~~~
                            AT_HWCAP

This adds back an explicit include for the header.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>