LU-17490 tests: update .gitignore

LU-17490 tests: update .gitignore

Otherwise, we'll see this monitor_lustrefs binary in the
build tree.

Fixes: 7101742 ("LU-17490 tests: verify fanotify works for lustre")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I129c12515e607e97ab42917220a439ebb1823e8c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55082
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>