sched: split sched.h into separate header files

arch/x86_64/pmap.c

@@ -4,6 +4,7 @@
 #include <kernel/pmap.h>
 #include <kernel/printk.h>
 #include <kernel/sched.h>
+#include <kernel/task.h>
 #include <kernel/types.h>
 #include <kernel/vm-object.h>
 #include <kernel/vm-region.h>