multimedia/qt6-multimedia: Mask X11-only headers

When the Qt stack is built with X11 disabled globally, certain private
headers should not be included in the final pkg-plist.

This does not affect the default settings, so not bumping PORTREVISON.

Reported by: angry_vincent on #freebsd-desktop
MFH: 2026Q3

(cherry picked from commit 7ae4bbc02b5b96936d4bef4bb2561d38b704def3)