shmem: fix W=1 build warnings with CONFIG_SHMEM=n

With W=1 and CONFIG_SHMEM=n, shmem.c functions have no prototypes so the
compiler emits warnings.

Link: https://lkml.kernel.org/r/20230206190850.4054983-1-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mark Hemment <markhemm@googlemail.com>
Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -33,6 +33,7 @@
 #include <linux/random.h>
 #include <linux/sched/signal.h>
 #include <linux/export.h>
+#include <linux/shmem_fs.h>
 #include <linux/swap.h>
 #include <linux/uio.h>
 #include <linux/hugetlb.h>
@@ -58,7 +59,6 @@ static struct vfsmount *shm_mnt;
 #include <linux/string.h>
 #include <linux/slab.h>
 #include <linux/backing-dev.h>
-#include <linux/shmem_fs.h>
 #include <linux/writeback.h>
 #include <linux/pagevec.h>
 #include <linux/percpu_counter.h>