add Gavin as code owner for DefinitelyTyped (#332)

Author: zengin

microsoft-graph.d.ts

@@ -5,6 +5,7 @@
 //                 Mustafa Zengin <https://github.com/zengin>
 //                 DeVere Dyett <https://github.com/ddyett>
 //                 Nikitha Udaykumar Chettiar <https://github.com/nikithauc>
+//                 Gavin Barron <https://github.com/gavinbarron>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.1