MAUSB: fix warning: no previous prototype for 'proto_register_wfa_oui' [-Wmissing-prototypes]

Change-Id: I86cc9f6f9cd50a6e081aabc10bce077281eb2d76
Reviewed-on: https://code.wireshark.org/review/4959
Reviewed-by: Sean Stalley <seanstalley@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>