Typescript definition update (#3320)
Typescript definition update (#3320)

Updates to pass typescript compiler(tsc) type checks, given the Gatsby's default layout in src/layouts/index.js.
mingaldrichgan/gatsby,fk/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,fk/gatsby,0x80/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,danielfarrell/gatsby,fk/gatsby,0x80/gatsby,0x80/gatsby,danielfarrell/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby,fabrictech/gatsby,fabrictech/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby