kassert.h: Explicitly include <sys/types.h> on _KERNEL

MFCing commit "kassert: Explicitly include <sys/_types.h>" (as
3772cb06205fd6bd) was not enough as, on stable/14 and contrary to main,
<sys/kassert.h> also declares a 'panicked' variable as a 'bool' when
_KERNEL is defined, so 'bool' needs to be defined in this case for
<sys/kassert.h> to be includable standalone.

This is a direct commit to stable/14.

Fixes: 64ca7e040ace ("queue(3): Debug macros: Finer control knobs, userland support")
Sponsored by: The FreeBSD Foundation