Hide the Linux specific parts behind check for headers

Summary:
Only build
  * `virtual_terminal.cpp` in the presence of `linux/vt.h`
  * `fbdev`-backend in the precense of `linux/fb.h`

Test Plan:

Reviewers: #kwin, #freebsd, graesslin, bcooksley

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6847