Add plumbing icon to flipper

Summary: Adds a reference to plumbing icon for plugins to use

Reviewed By: svaithia

Differential Revision: D34467457

fbshipit-source-id: 21d95f663309224742f4f93ac584998e1a470f28

desktop/static/icons.json

@@ -601,6 +601,9 @@
   "camcorder-live": [
     16
   ],
+  "plumbing": [
+    16
+  ],
   "app-facebook-circle": [
     16
   ],
@@ -619,4 +622,4 @@
   "caution-triangle-outline": [
     24
   ]
-}
+}