decklink: fix initialization fails in windows binary

There is no log of gst_decklink_com_thread () which initializes COM.
The initialization part is not valid with #ifdef MSC_VER.
Windows binaries are built with gcc.
As with other codes, it was avoidable by setting it to G_OS_WIN32
instead of MSC_VER.

https://bugzilla.gnome.org/show_bug.cgi?id=794652
1 file changed