mount: make __depopulate_roots_yard static

As these function is never used outside of criu/mount.c
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>

criu/mount.c

@@ -3179,7 +3179,7 @@ static int populate_mnt_ns(void)
 	return ret;
 }
 
-int __depopulate_roots_yard(void)
+static int __depopulate_roots_yard(void)
 {
 	int ret = 0;