Qt: force XCB platform so we do not crash if QT_QPA_PLATFORM is set
Qt: force XCB platform so we do not crash if QT_QPA_PLATFORM is set

(cherry picked from commit 9a76128f6c86db806ba54f39007db826a7f9e533)
Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2