AUTHORS: Fix duplicate author

George Katevenis had two names in git logs.  Add a mailmap entry
to force use of his full name.

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

Author: bwbarrett

.mailmap

@@ -130,3 +130,5 @@ Samuel K. Gutierrez <samuel@lanl.gov> <samuel@lanl.gov>
 Tomislav Janjusic <tomislavj@nvidia.com> Tomislavj Janjusic <tomislavj@nvidia.com>
 
 William P. LePera <lepera@us.ibm.com>
+
+George Katevenis <gkatev@ics.forth.gr>