selftests/fs/mount-notify: Fix compilation failure.

selftests/fs/mount-notify: Fix compilation failure.

Commit c6d9775c2066 ("selftests/fs/mount-notify: build with tools include
dir") introduces the struct __kernel_fsid_t to decouple dependency with
headers_install.  The commit forgets to define a macro for __kernel_fsid_t
and it will cause type re-definition issue.

Signed-off-by: Xing Guo <higuoxing@gmail.com>
Link: https://lore.kernel.org/20250813031647.96411-1-higuoxing@gmail.com
Acked-by: Amir Goldstein <amir73il@gmail.com>
Closes: https://lore.kernel.org/oe-lkp/202508110628.65069d92-lkp@intel.com
Signed-off-by: Christian Brauner <brauner@kernel.org>