CI: Force CONFIG_XEN_IBT in the buster-gcc-ibt test

CI: Force CONFIG_XEN_IBT in the buster-gcc-ibt test

buster-gcc-ibt is a dedicated test to run a not-yet-upstreamed compiler patch
which is relevant to CONFIG_XEN_IBT in 4.17 and later.

Force it on, rather than having 50% of the jobs not testing what they're
supposed to be testing.

Fixes: 5d59421815d5 ("x86: Use control flow typechecking where possible")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
[stefano: minor code style improvement]
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>