6lowpan : fix warning: no previous prototype for 'lowpan_context_free' [-Wmissing-prototypes]

Change-Id: I3fa66aa0713a0455639b04ecf129f48baa8c096f
Reviewed-on: https://code.wireshark.org/review/5718
Reviewed-by: Anders Broman <a.broman58@gmail.com>