etc: Avoid duplicate gss files installation

From Cirrus-CI
su user -c "/usr/libexec/flua tools/pkgbase/metalog_reader.lua \

-c /usr/obj/$(pwd -P)/${TARGET}.${TARGET_ARCH}/worldstage/METALOG"

error: ./etc/gss/mech file repeated with same meta: line 2127,40175
error: ./etc/gss/qop file repeated with same meta: line 2128,40176

Prior to 1b735665f45a the Heimdal versions were installed via

distribution:
${_+_}cd ${.CURDIR}/gss; ${MAKE} install

1b735665f45a added a conditional for MK_MITKRB5 but also added gss or
gss-krb5 to SUBDIRS, causing these files to be installed twice.

Fixes: 1b735665f45a ("etc/gss: Add MIT GSS glue")
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation