Fix installation of private headers generated by wayland-scanner

Private qtwayland headers were not installed at first build, since
qmake was ignoring unexisting files from the install target. It
required another run of qmake to have a proper Makefile generated.

The rules for generated headers need CONFIG = no_check_exist, so that
files get listed in the Makefile even if they do not exist yet (thanks
to Loïc Yhuel for the pointer).

Change-Id: I1a0278d629295a55a3ddcf5f8fb068a04ba5be47
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>