selftests: cgroup: add gitignore file

Add the executable 'test_memcontrol' to a .gitignore file.
Signed-off-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NShuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/cgroup/.gitignore

+test_memcontrol