Fix compilation if no private headers are available

src/plugins/debugger/qml/qml.pri

@@ -1,13 +1,12 @@
-HEADERS += \
-    $$PWD/qmlengine.h \
-    $$PWD/canvasframerate.h \
+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 \
+    SOURCES += \
+        $$PWD/qmlengine.cpp \
+        $$PWD/canvasframerate.cpp \
 
-QT += declarative
-
-FORMS +=
-
-RESOURCES +=
+    QT += declarative
+}