Consistently prefix all platform plugin libraries with "q".

This means the xcb plugin library will be named libqxcb.so instead of
libxcb.so, which doesn't clash with the system's libxcb.so. We need to
consistently apply this on all platforms for static linking to work.

Change-Id: I1640a7cae7b9846bbe62b19ab1c2c5bad7d02b4c
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>