Build QMLObserver and qmldump only in debug mode

The problem is that we actually should have both debug & release builds,
but we don't have detection at the moment which binary to use.
See QTCREATORBUG-2496

Reviewed-by: Thomas Hartmann

share/qtcreator/qml/qmlobserver/qmlobserver.pro

@@ -1,6 +1,10 @@
 TEMPLATE = app
 DEFINES += QMLOBSERVER
 
+### FIXME: only debug plugins are now supported.
+CONFIG -= release
+CONFIG += debug
+
 include(qml.pri)
 
 SOURCES += main.cpp