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>