Fixes a bug in the LIBDIR install path

Introduced by commit 10517652f65f05120432ea05883977e39ed7b8be.
It made libQtMimeTypes installation fail.

Change-Id: I7c512b5d29bcfa06896a2df71f98b9c58db06817
Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Reviewed-by: David Faure (KDE) <faure@kde.org>