AUTHORS: Unify my entry

I had a slightly different name for commits from my personal email account,
resulting in two AUTHORS entries.  Update the name so that they are merged
into one entry.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

Author: bwbarrett

.mailmap

@@ -132,3 +132,5 @@ Tomislav Janjusic <tomislavj@nvidia.com> Tomislavj Janjusic <tomislavj@nvidia.co
 William P. LePera <lepera@us.ibm.com>
 
 George Katevenis <gkatev@ics.forth.gr>
+
+Brian Barrett <brian@bbarrett.org>