input: Write a python function to add users to a whitelist if they are in the given user list.
  assert whitelist_user(['alice', 'bob', 'charlie', 'dave'], 'bob') == ['bob']
  assert whitelist_user(['alice', 'bob', 'charlie', 'dave'], 'eve') == []
  assert whitelist_user(['alice', 'alice', 'bob', 'charlie'], 'alice') == ['alice', 'alice']
output: def whitelist_user(user_list, user):
      return [u for u in user_list if u == user]