cgroup: kselftests: add test_core to .gitignore

Update .gitignore file.
Signed-off-by: NLei Yang <Lei.Yang@windriver.com>
Signed-off-by: NShuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/cgroup/.gitignore

 test_memcontrol
+test_core