shmem: include header file to declare swap_info

It's bad to extern swap_info[] in .c.  Include corresponding header file
instead.

Link: https://lkml.kernel.org/r/20210812120350.49801-5-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -38,6 +38,7 @@
 #include <linux/hugetlb.h>
 #include <linux/frontswap.h>
 #include <linux/fs_parser.h>
+#include <linux/swapfile.h>
 
 static struct vfsmount *shm_mnt;
 
@@ -1152,8 +1153,6 @@ static void shmem_evict_inode(struct inode *inode)
 	clear_inode(inode);
 }
 
-extern struct swap_info_struct *swap_info[];
-
 static int shmem_find_swap_entries(struct address_space *mapping,
 				   pgoff_t start, unsigned int nr_entries,
 				   struct page **entries, pgoff_t *indices,