exec.h: Move PS_STRINGS define to kernel-only section

The kern.ps_strings sysctl was introduced in r103767 and the last
use of PS_STRINGS in userspace code was removed in r297888.

PR: 208760 [exp-run]
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5933