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/matrix.py

@@ -42,6 +42,7 @@ if TYPE_CHECKING:
         url: str
         info: MediaInfo
         id: str
+        msgtype: str
 else:
     MediaInfo = None
     StickerInfo = None