qtwebkit-examples: add missing RDEPENDS for qmlplugins

qtwebkit-examples: add missing RDEPENDS for qmlplugins

Some examples require qtwebkit-qmlplugins (flickerview, youtubeview),
and this dependency is not automatically added. Without this patch,
and without manually adding qtwekbit-qmlplugins, we get errors like
this:

qrc:///flickrview.qml:42:1: module "QtWebKit" is not installed

(From meta-qt5 rev: ffe8a0e0ad7e8594ace8c36abb11503bd1668658)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>