ipc/shm.c: make compat_ksys_shmctl() static

Fix the following sparse warning:

ipc/shm.c:1335:6: warning: symbol 'compat_ksys_shmctl' was not declared.
Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200403063933.24785-1-yanaijie@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -1332,7 +1332,7 @@ static int copy_compat_shmid_from_user(struct shmid64_ds *out, void __user *buf,
 	}
 }
 
-long compat_ksys_shmctl(int shmid, int cmd, void __user *uptr, int version)
+static long compat_ksys_shmctl(int shmid, int cmd, void __user *uptr, int version)
 {
 	struct ipc_namespace *ns;
 	struct shmid64_ds sem64;