exec: include pagemap.h again to fix build

Fix compilation errors on avr32 and without CONFIG_SWAP, introduced by
ba92a43dbaee339cf5915ef766d3d3ffbaaf103c ("exec: remove some includes")

In file included from include/asm/tlb.h:24,
from fs/exec.c:55:
include/asm-generic/tlb.h: In function 'tlb_flush_mmu':
include/asm-generic/tlb.h:76: error: implicit declaration of function 'release_pages'
include/asm-generic/tlb.h: In function 'tlb_remove_page':
include/asm-generic/tlb.h:105: error: implicit declaration of function 'page_cache_release'
make[1]: *** [fs/exec.o] Error 1

This straightforward part-revert is nobody's favourite patch to address
the underlying tlb.h needs swap.h needs pagemap.h (but sparc won't like
that) mess; but appropriate to fix the build now before any overhaul.

Reported-by: Yoichi Yuasa <[e-mail]>
Reported-by: Haavard Skinnemoen <[e-mail]>
Signed-off-by: Hugh Dickins <[e-mail]>
Tested-by: Adrian Bunk <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/exec.c

···
 #include <linux/swap.h>
 #include <linux/string.h>
 #include <linux/init.h>
+#include <linux/pagemap.h>
 #include <linux/highmem.h>
 #include <linux/spinlock.h>
 #include <linux/key.h>