dependabot: Auto-approve PRs (solana-program#54)

#### Problem

Now that PRs require approval, dependabot PRs don't automatically merge.

#### Summary of changes

Add a step to approve the PR.

Author: joncinque

.github/workflows/dependabot-auto-merge.yml

@@ -15,3 +15,8 @@ jobs:
         env:
           PR_URL: ${{ github.event.pull_request.html_url }}
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Approve
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}