third_party/heimdal_build: Update fallthrough macro for switch statements

third_party/heimdal_build: Update fallthrough macro for switch statements

This is an adaptation to Heimdal:

commit 133f5174820b34e2a12c3f3412bf554cae2ee22f
Author: Daria Phoebe Brashear <dariaphoebe@auristor.com>
Date:   Fri Sep 16 09:57:24 2022 -0400

    rewrite fallthrough to HEIM_FALLTHROUGH to deal with new Apple SDKs

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Nov  2 05:21:29 UTC 2022 on sn-devel-184