Unbreak `make configure' after r457276.

Info: creating cache file /wrkdirs/usr/ports/net-im/teamwords/work/TeamWords-0.2.1/.qmake.cache
Project ERROR: Unknown module(s) in QT: webkitwidgets

I think the port was pulling qt5-webkit via qt5-assistant, and it broke after
qt5-assistant dropped the dependency.