sh: asm/tlb.h needs linux/swap.h

Commit 1e56a564

 introduced the mmu_gather
rework for sh, but missed a linux/swap.h include:

	CC      arch/sh/mm/tlb-urb.o
	In file included from arch/sh/mm/tlb-urb.c:14:0:
	arch/sh/include/asm/tlb.h: In function '__tlb_remove_page':
	arch/sh/include/asm/tlb.h:92:2: error: implicit declaration of function 'free_page_and_swap_cache'

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>