selftests/timens: add futex binary to .gitignore

Add the futex test binary introduced by commit a4fd8414
("selftests/timens: Add a test for futex()") to .gitignore.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>