QtVersion: Fix detection of system Qt

We used to get a list of possible qmake file names, but now we
just get a list of filename patterns.

Task-number: QTCREATORBUG-13200
Change-Id: I9e635ec04b25139cfa8e7096173ac9e1910d0ef5
Reviewed-by: Daniel Teske <daniel.teske@digia.com>