selftests: add missing set-tz to timers .gitignore

set-tz is missing from timers/.gitignore. Add it.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/timers/.gitignore

@@ -17,3 +17,4 @@ skew_consistency
 threadtest
 valid-adjtimex
 adjtick
+set-tz