mm/hugetlb.c must #include <asm/io.h>

This patch fixes the following build error on sh caused by
commit aa888a7497
(hugetlb: support larger than MAX_ORDER):

<--  snip  -->

...
  CC      mm/hugetlb.o
/home/bunk/linux/kernel-2.6/git/linux-2.6/mm/hugetlb.c: In function 'alloc_bootmem_huge_page':
/home/bunk/linux/kernel-2.6/git/linux-2.6/mm/hugetlb.c:958: error: implicit declaration of function 'virt_to_phys'
make[2]: *** [mm/hugetlb.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

mm/hugetlb.c

@@ -16,7 +16,7 @@
 #include <linux/mutex.h>
 #include <linux/bootmem.h>
 #include <linux/sysfs.h>
-
+#include <asm/io.h>
 #include <asm/page.h>
 #include <asm/pgtable.h>