[kernel] Don't double-construct the scheduler

The scheduler singleton was getting constructed twice, once at static
time and then again in main(). Make the singleton a pointer so we only
construct it once.

src/kernel/main.cpp

@@ -174,7 +174,7 @@ kernel_main(args::header *header)
 	devices.init_drivers();
 
 	syscall_enable();
-	scheduler *sched = new (&scheduler::get()) scheduler(devices.get_lapic());
+	scheduler *sched = new scheduler(devices.get_lapic());
 
 	sched->create_kernel_task(logger_task, scheduler::max_priority-1, true);