bsdinstall/pkgbase: Restore the kernel-dbg component

Commit 561dc357c2f5 changed the way we handle components in the prompt,
and accidentally resulted in the kernel-dbg component not being listed
since we now hide all components ending in "-dbg".

Add an exception for kernel-dbg to bring it back.

Fixes: 561dc357c2f5 ("bsdinstall: Use package sets for pkgbase install")
MFC after: 3 seconds
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52826