mac-decklink: Fix C++ virtual function warnings

Created another virtual function to override and moves stubbed function
to the header.

plugins/decklink/DecklinkBase.cpp

@@ -15,4 +15,8 @@ bool DecklinkBase::Activate(DeckLinkDevice *, long long)
 	return false;
 }
 
-void DecklinkBase::Deactivate() {}
+bool DecklinkBase::Activate(DeckLinkDevice *, long long, BMDVideoConnection,
+			    BMDAudioConnection)
+{
+	return false;
+}

plugins/decklink/DecklinkBase.h

@@ -27,7 +27,10 @@ protected:
 
 public:
 	virtual bool Activate(DeckLinkDevice *device, long long modeId);
-	virtual void Deactivate();
+	virtual bool Activate(DeckLinkDevice *device, long long modeId,
+			      BMDVideoConnection bmdVideoConnection,
+			      BMDAudioConnection bmdAudioConnection);
+	virtual void Deactivate() = 0;
 
 	DeckLinkDevice *GetDevice() const;
 };

plugins/decklink/DecklinkInput.hpp

@@ -41,8 +41,8 @@ public:
 
 	bool Activate(DeckLinkDevice *device, long long modeId,
 		      BMDVideoConnection bmdVideoConnection,
-		      BMDAudioConnection bmdAudioConnection);
-	void Deactivate();
+		      BMDAudioConnection bmdAudioConnection) override;
+	void Deactivate() override;
 	bool Capturing();
 
 	bool buffering = false;