configure: add SCHED_IDLE check

We can't consider it always available on Linux. It fails on
Debian 6.x, for instance.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/os/os-linux.h b/os/os-linux.h
index acc5a20..c45f071 100644
--- a/os/os-linux.h
+++ b/os/os-linux.h
@@ -35,7 +35,6 @@
 #define FIO_HAVE_TRIM
 #define FIO_HAVE_BINJECT
 #define FIO_HAVE_GETTID
-#define FIO_HAVE_SCHED_IDLE
 #define FIO_USE_GENERIC_INIT_RANDOM_STATE
 
 #ifdef MAP_HUGETLB
@@ -247,10 +246,12 @@
 	return errno;
 }
 
+#ifdef CONFIG_SCHED_IDLE
 static inline int fio_set_sched_idle(void)
 {
 	struct sched_param p = { .sched_priority = 0, };
 	return sched_setscheduler(gettid(), SCHED_IDLE, &p);
 }
+#endif
 
 #endif