sched: Don't include nuttx/sched.h inside sched.h

But let nuttx/sched.h include sched.h instead to
avoid expose nuttx kernel API to userspace.

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

drivers/usbmonitor/usbmonitor.c

@@ -30,8 +30,8 @@
 #include <stdbool.h>
 #include <unistd.h>
 #include <sched.h>
-#include <syslog.h>
 #include <errno.h>
+#include <debug.h>
 
 #include <nuttx/signal.h>
 #include <nuttx/kthread.h>