zdtm: sched_prio -- Add missing headers

On glibc-2.16 I'm getting

 | sched_prio00.c: In function ‘main’:
 | sched_prio00.c:48:3: error: implicit declaration of function ‘setpriority’ [-Werror=implicit-function-declaration]
 | sched_prio00.c:48:19: error: ‘PRIO_PROCESS’ undeclared (first use in this function)
 | sched_prio00.c:48:19: note: each undeclared identifier is reported only once for each function it appears in
 | sched_prio00.c:60:3: error: implicit declaration of function ‘getpriority’ [-Werror=implicit-function-declaration]

fix it adding headers needed.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>