cyrus-sasl: fix build with GCC 15.1

cyrus-sasl: fix build with GCC 15.1

Apply the patch Fedora uses, named cyrus-sasl-2.1.28-gcc15.patch there,
to fix building with GCC 15.1. As with other recent versions, GCC 15.1
transforms some warnings into errors, and this patch removes those
warnings, now errors. Upstream accepted a similar patch by the same
author at:

https://github.com/cyrusimap/cyrus-sasl/pull/869

The commit described here does not use the upstream patch because it
assumes the removal of CRAM-MD5 (lib/md5.c), but this has not happened
as of the packaged version, namely 2.1.28. A future release will clean
all of this up and remove the need for the patch this commit adds.

Signed-off-by: W. Michael Petullo <[email protected]>