boottrace: Add corresponding sysctl knob for loader tunable

The loader tunable 'kern.boottrace.table_size' does not have
corresponding sysctl MIB entry. Add it so that it can be retrieved,
and sysctl -T will also report it correctly.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42138