build: add missing @ngtools/webpack devDep

It is used by the publishing script, and was previously being added transitively via schematics (removed in angular/devkit#810).

Author: filipesilva

package-lock.json

@@ -56,6 +56,7 @@
     "yargs-parser": "^9.0.2"
   },
   "devDependencies": {
+    "@ngtools/json-schema": "^1.1.0",
     "@types/common-tags": "^1.2.4",
     "@types/express": "^4.0.32",
     "@types/fs-extra": "^4.0.0",