GSM a BSSMAP : fix warning: no previous prototype for 'dissect_bssmap' [-Wmissing-prototypes]

Change-Id: Ibc03864498cb88e93cb3f774d1f4987aa10dd32e
Reviewed-on: https://code.wireshark.org/review/5720
Reviewed-by: Anders Broman <a.broman58@gmail.com>