When compiling decklink, this header is included from
a C++ file (albeit inside 'extern "C"') and this
causes compilation failures because of an implicit
void* -> char* conversion. So add an explicit cast.
Fixes ticket #9819.
Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>