QmlJsInspector: remove private header dependencies

A copy of the client debugging code is made in src/libs/qmljsdebugclient/
(this comes from the qt code from commit 65642dd343bf61)

So the qmljsinspector plugin does not require anymore Qt private headers.

src/plugins/debugger/qml/qml.pri

@@ -1,11 +1,13 @@
+
+HEADERS += $$PWD/qmlengine.h
+SOURCES += $$PWD/qmlengine.cpp
+
 include(../../../private_headers.pri)
 exists($${QT_PRIVATE_HEADERS}/QtDeclarative/private/qdeclarativecontext_p.h) {
     HEADERS += \
-        $$PWD/qmlengine.h \
         $$PWD/canvasframerate.h \
 
     SOURCES += \
-        $$PWD/qmlengine.cpp \
         $$PWD/canvasframerate.cpp \
 
     QT += declarative