libqtprofilesetup: Read prl files also for Qt 4.

If we don't do that, nothing will work, because the qbs modules'
libFilePath property won't be set. Independent of that, it's the right
thing to do.

Change-Id: Ifa946281fd0e7b045bbd30bd166e661cabce7c1d
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>