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>

libavdevice/decklink_common.cpp

@@ -77,7 +77,7 @@ static IDeckLinkIterator *decklink_create_iterator(AVFormatContext *avctx)
     return iter;
 }
 
-int decklink_get_attr_string(IDeckLink *dl, BMDDeckLinkAttributeID cfg_id, const char **s)
+static int decklink_get_attr_string(IDeckLink *dl, BMDDeckLinkAttributeID cfg_id, const char **s)
 {
     DECKLINK_STR tmp;
     HRESULT hr;