avdevice/decklink: Suppress warning about misuse of struct instead of class

When building with Clang, the following warning is shown:

warning: struct 'IDeckLinkVideoFrame' was previously declared as a
class [-Wmismatched-tags]

The function incorrectly casts IDeckLinkVideoFrame as a struct
instead of a class pointer.
Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavdevice/decklink_enc.cpp

@@ -321,7 +321,7 @@ static int decklink_write_video_packet(AVFormatContext *avctx, AVPacket *pkt)
     pthread_mutex_unlock(&ctx->mutex);
 
     /* Schedule frame for playback. */
-    hr = ctx->dlo->ScheduleVideoFrame((struct IDeckLinkVideoFrame *) frame,
+    hr = ctx->dlo->ScheduleVideoFrame((class IDeckLinkVideoFrame *) frame,
                                       pkt->pts * ctx->bmd_tb_num,
                                       ctx->bmd_tb_num, ctx->bmd_tb_den);
     /* Pass ownership to DeckLink, or release on failure */