[PATCH] uml-i386: fix build breakage with CONFIG_HIGHMEM

missing helper used by arch/i386/mm/highmem.c, which is pulled
into build on that configuration.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>