add sched_{get,set}attr syscall numbers and SCHED_DEADLINE macro

linux 3.14 introduced sched_getattr and sched_setattr syscalls in
commit d50dde5a10f305253cbc3855307f608f8a3c5f73

and the related SCHED_DEADLINE scheduling policy in
commit aab03e05e8f7e26f51dee792beddcb5cca9215a5

but struct sched_attr "extended scheduling parameters data structure"
is not yet exported to userspace (necessary for using the syscalls)
so related uapi definitions are not added yet.

include/sched.h

@@ -38,6 +38,7 @@ int     sched_yield(void);
 #define SCHED_RR 2
 #define SCHED_BATCH 3
 #define SCHED_IDLE 5
+#define SCHED_DEADLINE 6
 #define SCHED_RESET_ON_FORK 0x40000000
 
 #ifdef _GNU_SOURCE