Fix missing __APPLE_USE_RFC_3542 (#12753)

* Fix missing __APPLE_USE_RFC_3542

#### Problem

In splitting `UDPEndPoint.cpp` by implementation, the definition
of `__APPLE_USE_RFC_3542` was lost, which caused `IPV6_PKTINFO`
to not be defined and corresponding parts of `UDPEndPoint` not to
be built.

#### Change overview

Add `#define __APPLE_USE_RFC_3542` to the top of
`UDPEndPointImplSockets.cpp`, as it had been in
`UDPEndPoint.cpp`.

#### Testing

CI

* Update src/inet/UDPEndPointImplSockets.cpp

Co-authored-by: Tennessee Carmel-Veilleux <tennessee.carmelveilleux@gmail.com>

* Fix trailing  whitespace

Co-authored-by: Andrei Litvin <andy314@gmail.com>
Co-authored-by: Tennessee Carmel-Veilleux <tennessee.carmelveilleux@gmail.com>

src/inet/UDPEndPointImplSockets.cpp

@@ -21,6 +21,9 @@
  * This file implements Inet::UDPEndPoint using sockets.
  */
 
+// Required to properly support underlying RFC3542-related fields to IPV6_PKTINFO
+// on Darwin.
+#define __APPLE_USE_RFC_3542
 #include <inet/UDPEndPointImplSockets.h>
 
 #include <lib/support/CodeUtils.h>