Add bors to .mailmap.

Before:
```
$ git shortlog -se | grep "\<bors\>"
 27755	bors <bors@rust-lang.org>
  5729	bors[bot] <26634292+bors[bot]@users.noreply.github.com>
   995	bors[bot] <bors[bot]@users.noreply.github.com>
```
After:
```
$ git shortlog -se | grep "\<bors\>"
 34479	bors <bors@rust-lang.org>
```

Author: nnethercote

.mailmap

@@ -73,6 +73,8 @@ Björn Steinbrink <bsteinbr@gmail.com> <B.Steinbrink@gmx.de>
 blake2-ppc <ulrik.sverdrup@gmail.com> <blake2-ppc>
 boolean_coercion <booleancoercion@gmail.com>
 Boris Egorov <jightuse@gmail.com> <egorov@linux.com>
+bors <bors@rust-lang.org> bors[bot] <26634292+bors[bot]@users.noreply.github.com>
+bors <bors@rust-lang.org> bors[bot] <bors[bot]@users.noreply.github.com>
 Braden Nelson <moonheart08@users.noreply.github.com>
 Brandon Sanderson <singingboyo@gmail.com> Brandon Sanderson <singingboyo@hotmail.com>
 Brett Cannon <brett@python.org> Brett Cannon <brettcannon@users.noreply.github.com>