Fix examples/widgets linking problems on QNX

Since QNX ld only supports staging-relative rpath values, the absolute
filesystem rpath stored in libQt5WebEngineWidgets.so doesn't help it
find libQtWebEngineCore.so.

As a QNX-specific counterpart to the rpath hack, we add an rpath-link
to qt_lib_webengine.pri that will allow webenginewidgets apps to find
the lib at link time.

Change-Id: I8217805edebebe55c66e47d6e9409c20f1dfc717
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>