avdevice/decklink_common: Fix Decklink for Mac

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/decklink_common.cpp

@@ -70,6 +70,16 @@ static char *dup_wchar_to_utf8(wchar_t *w)
 #define DECKLINK_STR    OLECHAR *
 #define DECKLINK_STRDUP dup_wchar_to_utf8
 #define DECKLINK_FREE(s) SysFreeString(s)
+#elif __APPLE__
+static char *dup_cfstring_to_utf8(CFStringRef w)
+{
+    char s[256];
+    CFStringGetCString(w, s, 255, kCFStringEncodingUTF8);
+    return av_strdup(s);
+}
+#define DECKLINK_STR    const __CFString *
+#define DECKLINK_STRDUP dup_cfstring_to_utf8
+#define DECKLINK_FREE(s) free((void *) s)
 #else
 #define DECKLINK_STR    const char *
 #define DECKLINK_STRDUP av_strdup