decklink: add default virtual destructor

The class has virtual functions and is calling `delete this;` so it
triggers a warning if the class has no virtual destructors.

    modules/access/decklink.cpp: In member function ‘virtual ULONG {anonymous}::DeckLinkCaptureDelegate::Release()’:
    modules/access/decklink.cpp:268:13: warning: deleting object of polymorphic class type ‘{anonymous}::DeckLinkCaptureDelegate’ which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
      268 |             delete this;
          |             ^~~~~~~~~~~