build QML plugins with relative RPATH as well

amends 967372c97.

Change-Id: I898950d6847e43d565748cd8d1ea583cf5ca5c9d
Reviewed-by: Jake Petroules <jake.petroules@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

mkspecs/features/qml_plugin.prf

@@ -19,6 +19,8 @@ if(win32|mac):!macx-xcode {
     contains(QT_CONFIG, build_all):CONFIG += build_all
 }
 
+CONFIG += relative_qt_rpath  # Qt's QML plugins should be relocatable
+
 !no_cxx_module:isEmpty(CXX_MODULE) {
     CXX_MODULE = $$TARGET
     TARGET = declarative_$${TARGET}