mavlink_main.cpp - add fix for Cygwin (Windows)

fix for issue https://github.com/PX4/Firmware/issues/11030 - "getting required buffer size failed" on jmavsim simulation on Windows 10

src/modules/mavlink/mavlink_main.cpp

@@ -941,7 +941,7 @@ Mavlink::find_broadcast_address()
 	struct ifconf ifconf;
 	int ret;
 
-#if defined(__APPLE__) && defined(__MACH__)
+#if defined(__APPLE__) && defined(__MACH__) || defined(__CYGWIN__)
 	// On Mac, we can't determine the required buffer
 	// size in advance, so we just use what tends to work.
 	ifconf.ifc_len = 1024;