[fontdatabase] Use the CONFIG += qpa/genericunixfontdatabase

[fontdatabase] Use the CONFIG += qpa/genericunixfontdatabase

Late follow up of fc9a648dc03ec4531553bc3d65ae4dba3c5b5980 that
tries to make plugins indifferent of where the source is located
in relation to qtbase.

Change-Id: I9a5944fe737056216c7174d8d9151c8d0e776a22
Merge-request: 64
Reviewed-on: http://codereview.qt-project.org/5933
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>