Display System Memory on FreeBSD too

Summary:
Kinfocenter currently only calculates a value for Linux, behind a Q_OS_LINUX.
Extend this with a Q_OS_FREEBSD. The same code would probably also work on the other BSD-flavours.

Reviewers: sebas, rakuco

Reviewed By: sebas

Differential Revision: https://phabricator.kde.org/D1201