CMake: Load plugin files unconditionally if present.

Task-number: QTBUG-39171
Task-number: QTBUG-39451
Change-Id: Ie66bd6b787a0957fd6f7ea673b158ab5de3cc38f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>