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/[e-mail]
Cc: Shuah Khan <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Miklos Szeredi <[e-mail]>
Cc: Josef Bacik <[e-mail]>
Reviewed-by: Charlie Jenkins <[e-mail]>
Tested-by: Charlie Jenkins <[e-mail]>
Signed-off-by: Li Zhijian <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/filesystems/statmount/.gitignore

···
 # SPDX-License-Identifier: GPL-2.0-only
+statmount_test_ns
 /*_test