uml: compile error fix

This patch fixes this error:

In file included from /home/wangcong/projects/linux-2.6/arch/um/kernel/smp.c:9:
include2/asm/tlb.h: In function `tlb_remove_page':
include2/asm/tlb.h:101: error: implicit declaration of function `page_cache_release'

And since including <linux/pagemap.h> in <linux/swap.h> will break sparc,
we add this #include in uml's own header.

Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: WANG Cong <wangcong@zeuux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>