Fix compilation if no private headers are available

src/plugins/debugger/qml/qml.pri

@@ -1,3 +1,5 @@
+include(../../../private_headers.pri)
+exists(${QT_PRIVATE_HEADERS}/QtDeclarative/private/qdeclarativecontext_p.h) {
     HEADERS += \
         $$PWD/qmlengine.h \
         $$PWD/canvasframerate.h \
@@ -7,7 +9,4 @@ SOURCES += \
         $$PWD/canvasframerate.cpp \
 
     QT += declarative
-
-FORMS +=
-
-RESOURCES +=
+}