Define _POSIX_PRIORITY_SCHEDULING as 0.

sched_*(2) syscalls might be not available at runtime. Defining this
constant as zero directs POSIX-compliant code to call sysconf(3) to
detect the feature at runtime, and forces libc sysconf(3) to ask
kernel.

Noted by: ngie
Reviewed by: jilles, ngie
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D9055