EGL convenience: Guard Linux-specific code with Q_OS_LINUX.

Instead of "blacklisting" Unix platforms that don't have Linux headers,
do the opposite and check for Q_OS_LINUX in the framebuffer code that
has Linux-specific code.

This should help fix the build on other Unices, such as the BSDs.

Change-Id: Icb6edf34bb20c9a98843b6dc0e2a87ee71fcd046
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>