Increase GraphQL benchmark iterations from 2 to 15

This is done to try and increase stability of the benchmark as the
numbers collected currently fluctuate a lot.

Author: watson

benchmark/sirun/plugin-graphql/meta.json

@@ -3,7 +3,7 @@
   "run": "node index.js",
   "run_with_affinity": "bash -c \"taskset -c $CPU_AFFINITY node index.js\"",
   "cachegrind": false,
-  "iterations": 2,
+  "iterations": 15,
   "instructions": true,
   "variants": {
     "control": {},