selftests mount: Fix mount_setattr_test builds failed

When compiling selftests with target mount_setattr I encountered some errors with the below messages:
mount_setattr_test.c: In function ‘mount_setattr_thread’:
mount_setattr_test.c:343:16: error: variable ‘attr’ has initializer but incomplete type
  343 |         struct mount_attr attr = {
      |                ^~~~~~~~~~

These errors might be because of linux/mount.h is not included. This patch resolves that issue.
Signed-off-by: Anh Tuan Phan <tuananhlfc@gmail.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/mount_setattr/mount_setattr_test.c

@@ -18,6 +18,7 @@
 #include <grp.h>
 #include <stdbool.h>
 #include <stdarg.h>
+#include <linux/mount.h>
 
 #include "../kselftest_harness.h"