message: Add modp_b64 to thirdparty.json
diff: "url": "https://github.com/okdshin/PicoSHA2",
  "license": "MIT"
  },
  + {
  + "name": "modp_b64",
  + "url": "https://github.com/chromium/chromium/tree/15996b5d2322b634f4197447b10289bddc2b0b32/third_party/modp_b64",
  + "license": "BSD-3-Clause"
  + },
  {
  "name": "RapidJSON",
  "url": "https://github.com/Tencent/rapidjson",
  