Ensure qconfig-modules.h does not include qconfig-webkit.h

r448361 updated Qt WebKit and dropped QT_DIST and QT_DEFINES, which led to
existing installations being upgraded to remove qconfig-webkit.h while not
removing the corresponding #include <QtCore/modules/qconfig-webkit.h> from
qconfig-modules.h.

Consequently, any build that uses existing Qt5 ports (i.e. building a port
locally, or any project using Qt5) fails with errors like:

/usr/local/include/qt5/QtCore/qconfig-modules.h:8:10: fatal error:
'QtCore/modules/qconfig-webkit.h' file not found

Add a pkg-install that takes care of removing any offending lines from
qconfig-modules.h.

PR: 221828
Reviewed by: tcberner, Adriaan de Groot <groot@kde.org>