I've struck an incredibly frustrating problem when attempting to build KF5. The whole thing counts on a module of Qt5 called QtWebkitWidgets that does not appear in the Qt5 sources. I've been busy looking for it around the web and aside from the documentation there is no evidence (source code) anywhere that I can find.

Can someone tell me how to get it?

When you compiled Qt 5, did it build Webkit as well? In any case, please check your build logs for Qt 5 to ensure that the Webkit build process did not complain about any missing dependencies (this won't appear in the main Qt configure output).