selftests: add gpio generated files to .gitignore

gpio test generates files in selftests directory. Add them to .gitignore.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/.gitignore

 kselftest
+gpiogpio-event-mon
+gpiogpio-hammer
+gpioinclude/
+gpiolsgpio