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