selftests/mount_settattr: add missing STATX_MNT_ID_UNIQUE define

selftests/mount_settattr: add missing STATX_MNT_ID_UNIQUE define

  CC       mount_setattr_test
In file included from mount_setattr_test.c:24:
mount_setattr_test.c: In function ‘mount_setattr_mount_detached_mount_on_detached_mount_and_attach’:
mount_setattr_test.c:1850:60: error: ‘STATX_MNT_ID_UNIQUE’ undeclared (first use in this function); did you mean ‘STATX_MNT_ID’?
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |                                                            ^~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:757:20: note: in definition of macro ‘__EXPECT’
  757 |         __typeof__(_expected) __exp = (_expected); \
      |                    ^~~~~~~~~
mount_setattr_test.c:1850:9: note: in expansion of macro ‘ASSERT_EQ’
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |         ^~~~~~~~~
mount_setattr_test.c:1850:60: note: each undeclared identifier is reported only once for each function it appears in
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |                                                            ^~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:757:20: note: in definition of macro ‘__EXPECT’
  757 |         __typeof__(_expected) __exp = (_expected); \
      |                    ^~~~~~~~~
mount_setattr_test.c:1850:9: note: in expansion of macro ‘ASSERT_EQ’
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |         ^~~~~~~~~

Signed-off-by: Christian Brauner <brauner@kernel.org>