Add some codeowners to Concurrency directories

I'd love to be notified for PRs in those without grepping PR titles so proposing to add myself here. 

More than happy to include more folks? Would it make sense to include e.g. @hborla @DougGregor ?

Author: ktoso

.github/CODEOWNERS

@@ -139,13 +139,15 @@
 # stdlib
 # TODO: /stdlib/
 /stdlib/public/Backtracing/               @al45tair
+/stdlib/public/Concurrency/               @ktoso
 /stdlib/public/Cxx/                       @zoecarver @hyp @egorzhdan
 /stdlib/public/Distributed/               @ktoso
 /stdlib/public/Windows/                   @compnerd
 /stdlib/public/libexec/swift-backtrace/   @al45tair
 
 # test
 /test/ASTGen/                                       @zoecarver @CodaFi
+/test/Concurrency/                                  @ktoso
 /test/Constraints/                                  @hborla @xedin
 /test/DebugInfo/                                    @adrian-prantl
 /test/Distributed/                                  @ktoso