add piwik config to riot.im electron build config

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

Author: t3chguy

electron_app/riot.im/config.json

@@ -12,5 +12,9 @@
         "servers": [
             "matrix.org"
         ]
+    },
+    "piwik": {
+        "url": "//piwik.riot.ovh/",
+        "siteId": 2
     }
 }