libprocstat: Drop all _KERNEL defines outside of zfs_defs.c

Defining _KERNEL is a historical hack that can often break due to the
environment not actually being that of a kernel build. Now that we have
other targeted macros we can define instead that don't have far-reaching
implications like _KERNEL we can drop this.

Reviewed by: imp, kib, markj
Differential Revision: https://reviews.freebsd.org/D50719