decklink: Fix mingw compiler warnings

plugins/decklink/decklink-device-instance.cpp

@@ -19,6 +19,10 @@ DeckLinkDeviceInstance::DeckLinkDeviceInstance(DeckLink *decklink_,
 	currentPacket.format          = AUDIO_FORMAT_16BIT;
 }
 
+DeckLinkDeviceInstance::~DeckLinkDeviceInstance()
+{
+}
+
 void DeckLinkDeviceInstance::HandleAudioPacket(
 		IDeckLinkAudioInputPacket *audioPacket,
 		const uint64_t timestamp)

plugins/decklink/decklink-device-instance.hpp

@@ -19,6 +19,7 @@ protected:
 
 public:
 	DeckLinkDeviceInstance(DeckLink *decklink, DeckLinkDevice *device);
+	virtual ~DeckLinkDeviceInstance();
 
 	inline DeckLinkDevice *GetDevice() const {return device;}
 	inline long long GetActiveModeId() const