Add missing errno.h include

Change-Id: I17655fbd86aee6b135bd02df140dd4cd2e253a43
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Allow to set a fixed DPI for the screen

When the QT_WAYLAND_FORCE_DPI environment vairable is set use that value
for Dpi.

Needed in some cases for font matching.

Change-Id: Ia02ec4743894d41a18471e6cbcca764da45a6844
Reviewed-by: Andy Nichols <andy.nichols@digia.com>