selftests/fs/statmount: build with tools include dir

selftests/fs/statmount: build with tools include dir

Copy the required headers files (mount.h, nsfs.h) to the tools
include dir and define the statmount/listmount syscall numbers
to decouple dependency with headers_install for the common cases.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/20250509133240.529330-3-amir73il@gmail.com
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>