exec: include pagemap.h again to fix build

Fix compilation errors on avr32 and without CONFIG_SWAP, introduced by
ba92a43 ("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 <[email protected]>
Reported-by: Haavard Skinnemoen <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Tested-by: Adrian Bunk <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/exec.c

@@ -32,6 +32,7 @@
 #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>