ssh: use the defaultkey sentinel for the ML-DSA-44 default host key

[ojanerif_amd.com]

Summary

servconf_add_hostkey() compares its file argument to defaultkey by pointer
to skip an unreadable default key; the ML-DSA-44 entry passed the "[default]"
literal, so the skip (and the "No host key files found" fallback) never
applied to it.

MFC after: 1 week

[emaste]

Fixes: bb5c77e9d281 ("OpenSSH: Update to 10.4p1")