add missing msgtype = m.sticker

On iOS the message is sent twice, with a duplicate event_id.
It cause error on logs, in different places (synapse, mautrix, ...)

It required to fix the already existing json or reimport the stickers.

The "packs/scalar*" example already include this field, and it works.

sticker/lib/util.py

@@ -74,4 +74,5 @@ def make_sticker(mxc: str, width: int, height: int, size: int,
                 "mimetype": "image/png",
             },
         },
+        "msgtype": "m.sticker",
     }