fix shadow build with -qt-zlib and pre-generated headers
fix shadow build with -qt-zlib and pre-generated headers

Task-number: QTBUG-32641
Change-Id: I4d0b4df50b652cf47e5f0b75b6d3c1f714414007
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

kobolabs/qtimageformats,kobolabs/qtimageformats,kobolabs/qtimageformats,kobolabs/qtimageformats