Deploy QML plugin type information to correct directory.

This adds the plugins.qmltypes file to the list of files that are
deployed to the qml/QtWebKit directory.

This is needed e.g. for auto-completion in Creator.

Patch co-authored with Fawzi Mohamed.

Task-number: QTBUG-39013
Change-Id: Icce9a168a0558652b810ad16c89895e24a6fc6ef
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Merge branch 'stable' into st