sys/sysctl.h: Add missing <sys/kassert.h> include

Hopefully, 'sys/sysctl.h' is really self-contained now.

This fixes the GENERIC-NODEBUG compilation error in 'hwpstate_common.c'
after commit 804329587508 (see below).

Fixes: 1a446f765d9c ("sys/sysctl.h: Make it self-contained when included from the kernel")
Fixes: 804329587508 ("hwpstate{_amd,intel}(4): Move common knobs to a separate file")
Sponsored by: The FreeBSD Foundation