install_prereq: Add GMime 3.0.

Ubuntu 20.10 does not come with GMime 2.6. Ubuntu 16.04 LTS does not
come with GMime 3.0. aptitude ignores any missing package. Therefore,
it installs the correct package(s). However, in Ubuntu 18.04 LTS and
Ubuntu 20.04 LTS, both versions are installed alongside although only
one is really needed.

Change-Id: Ic58aa9f2e131d94671f286f17dbd61e1ccbabcb7