sched: split sched.h into separate header files

syscall/log.c

@@ -1,5 +1,6 @@
 #include <kernel/printk.h>
 #include <kernel/sched.h>
+#include <kernel/task.h>
 
 kern_status_t sys_kern_log(const char *s)
 {

syscall/msg.c

@@ -3,6 +3,7 @@
 #include <kernel/printk.h>
 #include <kernel/sched.h>
 #include <kernel/syscall.h>
+#include <kernel/task.h>
 #include <kernel/vm-region.h>
 
 kern_status_t sys_channel_create(unsigned int id, kern_handle_t *out)

syscall/task.c

@@ -2,6 +2,8 @@
 #include <kernel/printk.h>
 #include <kernel/sched.h>
 #include <kernel/syscall.h>
+#include <kernel/task.h>
+#include <kernel/thread.h>
 #include <kernel/vm-region.h>
 
 extern kern_status_t sys_task_exit(int status)