add types files to cleaning script

Author: lobsterkatie

packages/gatsby/package.json

@@ -49,7 +49,7 @@
     "build:types:watch": "tsc -p tsconfig.types.json --watch",
     "build:npm": "ts-node ../../scripts/prepack.ts && npm pack ./build",
     "circularDepCheck": "madge --circular src/index.ts",
-    "clean": "rimraf build coverage",
+    "clean": "rimraf build coverage *.d.ts",
     "fix": "run-s fix:eslint fix:prettier",
     "fix:eslint": "eslint . --format stylish --fix",
     "fix:prettier": "prettier --write \"{src,test,scripts}/**/*.ts\"",