Add missing network.internal to v3 schema.

Signed-off-by: Daniel Nephin <[email protected]>

compose/config/config_schema_v3.0.json

@@ -308,6 +308,7 @@
           },
           "additionalProperties": false
         },
+        "internal": {"type": "boolean"},
         "labels": {"$ref": "#/definitions/list_or_dict"}
       },
       "additionalProperties": false