Make IPv6 pktinfo work on macOS

Define __APPLE_USE_RFC_3542 to expose the definition of IPV6_PKTINFO
on macOS.

https://github.com/krb5/krb5/commit/253e4459f4e4c9239e7a129bab4257f077d7bb94
Author: Greg Hudson <ghudson@mit.edu>
Commit: 253e4459f4e4c9239e7a129bab4257f077d7bb94
Branch: master
 src/lib/apputils/udppktinfo.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)