Fix compilation if no private headers are available

src/plugins/debugger/qml/qml.pri

-HEADERS += \
-    $$PWD/qmlengine.h \
-    $$PWD/canvasframerate.h \
-
-SOURCES += \
-    $$PWD/qmlengine.cpp \
-    $$PWD/canvasframerate.cpp \
-
-QT += declarative
-
-FORMS +=
-
-RESOURCES +=
+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
+}