GSM bssmap le: fix warning: no previous prototype for 'dissect_bssmap_le' [-Wmissing-prototypes]

Change-Id: Ifa8c7a15dfb37cf1dbd2f539dd0bbc87e6da1aea
Reviewed-on: https://code.wireshark.org/review/5719
Reviewed-by: Anders Broman <a.broman58@gmail.com>