Add Doug Gregor as code owner (#207)

Doug is taking on review responsibility for this repo, so adding
him to the auto-review list ensures PRs route to him alongside
the existing owners.

Author: rauhul

.github/CODEOWNERS

@@ -1,19 +1 @@
-# This file is a list of the people responsible for ensuring that patches for a
-# particular part of Swift are reviewed, either by themselves or by someone else.
-# They are also the gatekeepers for their part of Swift, with the final word on
-# what goes in or not.
-#
-# The list is sorted by surname and formatted to allow easy grepping and
-# beautification by scripts.  The fields are: name (N), email (E), web-address
-# (W), PGP key ID and fingerprint (P), description (D), and snail-mail address
-# (S).
-
-# N: Kuba Mracek
-# E: mracek@apple.com
-# D: Everything in swift-embedded-examples not covered by someone else
-
-###
-
-# The following lines are used by GitHub to automatically recommend reviewers.
-
-* @kubamracek @rauhul
+* @kubamracek @rauhul @douggregor