devel/gitlab-shell: disable GSSAPI support

It currently has a conflict between heimdal this port uses
and krb5 which is required by rubygem-timfel-krb5.
the heimdal port and krb5 as installing conflict binaries and
can therefor not exist both.

Problem is reported upstream:
https://gitlab.com/gitlab-org/gitlab/-/issues/405678

(cherry picked from commit bb2923418ee7549ccd24eafbc6f7a78e12307c6f)
(cherry picked from commit b5a874168ae078116b48f335b35967b31ff29be6)