Typescript definition update (gatsbyjs#3320)

Updates to pass typescript compiler(tsc) type checks, given the Gatsby's default layout in src/layouts/index.js.

Author: denster

packages/gatsby-link/index.d.ts

@@ -4,10 +4,11 @@ import { NavLinkProps } from "react-router-dom";
 export interface GatsbyLinkProps extends NavLinkProps {
   onClick?: (event: any) => void
   className?: string
+  style?:any;
 }
 
 export const navigateTo: (path: string) => void;
 
 export const withPrefix: (path: string) => string;
 
-export default class GatsbyLink extends React.Component<GatsbyLinkProps> { }
+export default class GatsbyLink extends React.Component<GatsbyLinkProps, any> { }