avdevice/decklink_common: Use defined(__APPLE__) instead of __APPLE__

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

libavdevice/decklink_common.cpp

@@ -70,7 +70,7 @@ 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__
+#elif defined(__APPLE__)
 static char *dup_cfstring_to_utf8(CFStringRef w)
 {
     char s[256];