Github 581 - Upgrade doctest

 * Upgrade doctest to version 2.4.6 to allow Arm NN to build with
   glibc 2.34

Signed-off-by: Jan Eilers <jan.eilers@arm.com>
Change-Id: Iff2ef286ba5ad0e0ec0f54f510253a2ec4b464d3
diff --git a/third-party/doctest/LICENSE.txt b/third-party/doctest/LICENSE.txt
index a204721..d67bb64 100644
--- a/third-party/doctest/LICENSE.txt
+++ b/third-party/doctest/LICENSE.txt
@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2016-2019 Viktor Kirilov
+Copyright (c) 2016-2021 Viktor Kirilov
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal