tslog: Add CTLFLAG_SKIP to sysctls

The timestamp logs are quite large (often much larger than all the
other sysctls combined) so it's unlikely anyone will want to have
them displayed by sysctl -a.

Differential Revision: https://reviews.freebsd.org/D34616

(cherry picked from commit 2406867f5b178a81c653b14524c0d5bad8189d2c)