avdevice/decklink: Fix compilation of module on OSX

avdevice/decklink: Fix compilation of module on OSX

Clang applies the missing-prototypes warning on C++ files, whereas
gcc only applies it to C.  As a result, the decklink_common.cpp file
fails to build because of missing prototypes in DecklinkDispatch.cpp
(which is #included by decklink_common.cpp).

We don't want to change the actual Blackmagic SDK sources, so
suppress the warning just for that one #include.

Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>