graphics/xpdf4: fix build if qt6 installed

Temporary workaround with turn off qt6 search before upstream fixed qt6 support.
Upstream aimed this in 4.05: https://forum.xpdfreader.com/viewtopic.php?t=42395

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D43295
MFH: 2024Q1