devel/qt6-base: pacify check-plist after 66b0e0515e2a

$ poudriere testport -j 131amd64 devel/qt6-base
[...]

> Checking for pkg-plist issues (check-plist)

> Parsing plist

> Checking for items in STAGEDIR missing from pkg-plist

Error: Orphaned: %%QT_DOCDIR%%/config/exampleurl-qtpositioning.qdocconf

> Checking for items in pkg-plist which are not in STAGEDIR

> Error: Plist issues found.

• Error code 1