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:
gitlab-org/gitlab#405678

(cherry picked from commit bb292341)
(cherry picked from commit b5a87416)