third_party/heimdal_build: Define fallthrough macro for switch statements

third_party/heimdal_build: Define fallthrough macro for switch statements

This is an adaptation to Heimdal:

commit ddc61136100b32346c4c4efa2bb6ddb5baedfb3e
Author: Nicolas Williams <nico@twosigma.com>
Date:   Fri Jan 14 16:32:04 2022 -0600

    Use fallthrough statement attribute

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14995

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>