update .gitignore in selftests/timers

Signed-off-by: Yuan Sun <sunyuan3@huawei.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/timers/.gitignore

@@ -16,3 +16,4 @@ set-timer-lat
 skew_consistency
 threadtest
 valid-adjtimex
+adjtick