Support Ubuntu Bionic arm64

[nurse]

We are planing to migrate from xenial (Ubuntu 16.04) to bionic(18.04).
This PR changes target Ubuntu version from xenial to bionic.
Additionally this includes some dependency fix to achieve more version independent code.

docker build -t ossec-hids-agent-bionic .
docker run --rm -it --privileged -v $PWD/result:/mnt ossec-hids-agent-bionic /bin/bash
root@63c2ba104002:/# /home/ubuntu/ossec-hids/contrib/debian-packages/generate_ossec.sh -d
root@63c2ba104002:/# /home/ubuntu/ossec-hids/contrib/debian-packages/generate_ossec.sh -u
root@63c2ba104002:/# /home/ubuntu/ossec-hids/contrib/debian-packages/generate_ossec.sh -b
root@63c2ba104002:/# cp /var/cache/pbuilder/bionic-arm64/result/ossec-hids-agent/ossec-hids-agent_3.6.0-1bionic_arm64.deb /mnt/
root@63c2ba104002:/# exit
ls result/*.deb
result/ossec-hids-agent_3.6.0-1bionic_arm64.deb

NOTE: Travis's arm64 CI takes long now but they plan to provider faster machines: https://blog.travis-ci.com/AWS-Graviton-2-support-comes-to-Travis-CI