Add debug file extension to kldxref(8) after r288176

After r288176 kernel debug files have the extension .debug. They also
moved to /usr/lib/debug/boot/kernel by default so in the normal case
kldxref does not encounter them. A src.conf(5) setting may be used to
continue installing them in /boot/kernel though, so have kldxref skip
.debug files in addition to .symbols files.

Reported by: fabient
Sponsored by: The FreeBSD Foundation