www/qt6-webengine: fix build on armv6/armv7 (again)

Re-add an extra file to the build that was accidentally deleted in a
previous update to the port.

Approved by: portmgr (build fix blanket)
Fixes: 9a0a6ca38c78e60b3afc10152daad948a1f001c7
MFH: 2023Q4
PR: 272422

(cherry picked from commit beac5673f03e9817d91dc91290c8acd66fb70445)