lib/upnp/Callback: make "evp" parameter const

src/lib/upnp/Callback.hxx

@@ -40,7 +40,7 @@ public:
 		return *(UpnpCallback *)cookie;
 	}
 
-	virtual int Invoke(Upnp_EventType et, void *evp) = 0;
+	virtual int Invoke(Upnp_EventType et, const void *evp) = 0;
 };
 
 #endif

src/lib/upnp/Discovery.cxx

@@ -182,7 +182,7 @@ UPnPDeviceDirectory::OnByeBye(const Upnp_Discovery *disco)
 // Example: ContentDirectories appearing and disappearing from the network
 // We queue a task for our worker thread(s)
 int
-UPnPDeviceDirectory::Invoke(Upnp_EventType et, void *evp)
+UPnPDeviceDirectory::Invoke(Upnp_EventType et, const void *evp)
 {
 	switch (et) {
 	case UPNP_DISCOVERY_SEARCH_RESULT:

src/lib/upnp/Discovery.hxx

@@ -157,7 +157,7 @@ private:
 	int OnByeBye(const Upnp_Discovery *disco);
 
 	/* virtual methods from class UpnpCallback */
-	virtual int Invoke(Upnp_EventType et, void *evp) override;
+	virtual int Invoke(Upnp_EventType et, const void *evp) override;
 };