Add captype to the CMake options so it gets built if required

Change-Id: I915a7caa32a1f644c30c1579a8ad15e39db9e02a
Reviewed-on: https://code.wireshark.org/review/3799
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>