avdevice/decklink: Fix compile breakage on OSX

Make the function static, or else Clang complains with:

error: no previous prototype for function 'decklink_get_attr_string' [-Werror,-Wmissing-prototypes]

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