Debugger: Fix compilation if no private headers are specified

Don't try to compile the qml debugger backend (which needs
private headers from Qt) if private headers cannot be located.

Reviewed-by: hjk

src/plugins/debugger/debugger.pro

@@ -112,8 +112,13 @@ include(cdb/cdb.pri)
 include(gdb/gdb.pri)
 include(script/script.pri)
 include(pdb/pdb.pri)
-include(qml/qml.pri)
 include(tcf/tcf.pri)
 include(shared/shared.pri)
 
+include(../../private_headers.pri)
+exists($${QT_PRIVATE_HEADERS}/QtDeclarative/private/qdeclarativecontext_p.h) {
+  DEFINES += QML_ENGINE=1
+  include(qml/qml.pri)
+}
+
 OTHER_FILES += Debugger.pluginspec