selftests/filesystems: Add missing gitignore file

Compiled binary files should be added to .gitignore

'git status' complains:
Untracked files:
(use "git add <file>..." to include in what will be committed)
     filesystems/statmount/statmount_test_ns

Link: https://lore.kernel.org/r/20241211004947.5806-1-lizhijian@fujitsu.com


Cc: Shuah Khan <shuah@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>