Add Lockfile to Fix Gatsby Example (vercel#4771)

This adds the Lockfile from Gatsby's official starter to fix deployments.

```
❯ npx gatsby-cli new ~/Desktop/scratch/test-default
❯ cp ~/Desktop/scratch/test-default/yarn.lock examples/gatsby
❯ cd examples/gatsby
❯ yarn
❯ git add examples/gatsby/yarn.lock
```