decklinkaudiosink: Fix compiler warning on OS X

decklinkaudiosink: Fix compiler warning on OS X

While gint64 and int64_t are always the same, clang does not agree with that.

/Applications/Xcode.app/Contents/Developer/usr/bin/make -C decklink
  CXX      libgstdecklink_la-gstdecklinkaudiosink.lo
gstdecklinkaudiosink.cpp:675:79: error: cannot initialize a parameter of type 'int64_t *' (aka 'long long *') with an rvalue of type 'gint64 *' (aka 'long *')
      ret = buf->output->attributes->GetInt (BMDDeckLinkMaximumAudioChannels, &max_channels);
                                                                              ^~~~~~~~~~~~~
./linux/DeckLinkAPI.h:692:87: note: passing argument to parameter 'value' here
    virtual HRESULT GetInt (/* in */ BMDDeckLinkAttributeID cfgID, /* out */ int64_t *value) = 0;
                                                                                      ^