MacOS - legacy versions: add fcntl.h to prevent undefined symbols for config change detection

src/lib/ares_event_configchg.c

@@ -260,6 +260,7 @@ done:
 #  include <unistd.h>
 #  include <notify.h>
 #  include <dlfcn.h>
+#  include <fcntl.h>
 
 struct ares_event_configchg {
   int fd;