`exec` should be added as a reserved word to avoid a breaking change in osconfig

[parthea]

In PR #1575, exec was removed from the reserved names list. Removing it from the list would result in a breaking change for osconfig. See exec_ here which would change to exec if we don't have exec as a reserved term . https://github.com/googleapis/python-os-config/blob/efa2c169e364e3e8eca62e4c06c14f44ab47db65/google/cloud/osconfig_v1/types/os_policy.py#L151