selftests mount: Fix mount_setattr_test builds failed

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>