Add missing ajv dependency to clusterio/lib

The lib/schema module uses ajv for compiling JSON Schema validators but
for some reason ajv was not part of the dependency list in its
package.json.

packages/lib/package.json

@@ -11,6 +11,7 @@
     "node": ">=10.16 <11 || >= 11.13"
   },
   "dependencies": {
+    "ajv": "^6.12.6",
     "fs-extra": "^8.1.0",
     "ini": "^1.3.5",
     "isomorphic-ws": "^4.0.1",