avdevice/decklink_common: Fix "Cross-compiling FFmpeg on Debian for Windows with MinGW-w64"

Fixes Ticket4130

Requested and Tested by: Zeranoe
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge pull request #1 from FFmpeg/master

avdevice/decklink_common: Fix "Cross-compiling FFmpeg on Debian for Wind...