samples/cgroup: add .gitignore file for generated samples

Make 'git status' quietly happy again after a full allmodconfig build.

Fixes: 60433a9d ("samples: introduce new samples subdir for cgroup")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

samples/cgroup/.gitignore

+/cgroup_event_listener
+/memcg_event_listener
+