Explicitly depend on www/qt5-webkit.
The dependency was being pulled indirectly (qt5-linguist -> qt5-assistant ->
qt5-webkit), which hasn't worked since r457276:
Reading /wrkdirs/usr/ports/sysutils/pcbsd-utils-qt5/work/pcbsd-3c4d2b/src-qt5/pc-softweb/pc-softweb.pro
Project ERROR: Unknown module(s) in QT: webkitwidgets