[kernel][warnings] fix a few -Wmissing-declarations warnings in the kernel

top/main.c

@@ -14,6 +14,7 @@
 
 #include <app.h>
 #include <arch.h>
+#include <kernel/init.h>
 #include <kernel/mutex.h>
 #include <kernel/novm.h>
 #include <kernel/thread.h>
@@ -40,8 +41,6 @@ static uint secondary_bootstrap_thread_count;
 
 static int bootstrap2(void *arg);
 
-extern void kernel_init(void);
-
 static void call_constructors(void) {
     void **ctor;