Don't embded qmldbg_tcp plugin to libQt5Qml in static build

Embedding qmldbg_tcp sources to libQt5Qml causes multipled
definitions of QTcpServerConnection symbols with static
build on Qt Quick 2 applications. Qmake can resolve
dependencies to static plugins applications use, so no
need to embed this to libQt5Qml.

Change-Id: I18c5e44b9ac3de4ef8be29cc5944de3527566b3c
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>