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>

tools/testing/selftests/timens/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 clock_nanosleep
 exec
+futex
 gettime_perf
 gettime_perf_cold
 procfs