D-Bus libdbus: avoid compiler warning

One method was still virtual, from the time when the class had a base
class. Remove the "virtual" keyword because a) it causes a compiler
warning in recent g++ about a class with non-virtual destructor with a
virtual method and b) it makes the generated code unnecessarily
complex.

src/gdbus/gdbus-cxx-bridge.h

@@ -1772,7 +1772,7 @@ class Watch : private boost::noncopyable
     {
     }
 
-    virtual void setCallback(const boost::function<void (void)> &callback)
+    void setCallback(const boost::function<void (void)> &callback)
     {
         m_callback = callback;
         if (m_called && m_callback) {