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

kernel/include/kernel/init.h

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2021 Travis Geiselbrecht
+ *
+ * Use of this source code is governed by a MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT
+ */
+#pragma once
+
+void kernel_init(void);
+

kernel/init.c

@@ -5,6 +5,8 @@
  * license that can be found in the LICENSE file or at
  * https://opensource.org/licenses/MIT
  */
+#include <kernel/init.h>
+
 #include <kernel/debug.h>
 #include <kernel/mp.h>
 #include <kernel/port.h>

kernel/novm/novm.c

@@ -152,7 +152,7 @@ static void novm_init(uint level) {
 LK_INIT_HOOK(novm, &novm_init, LK_INIT_LEVEL_PLATFORM_EARLY - 1);
 #endif
 
-void *novm_alloc_helper(struct novm_arena *n, size_t pages) {
+static void *novm_alloc_helper(struct novm_arena *n, size_t pages) {
     if (pages == 0 || pages > n->pages)
         return NULL;
 

kernel/thread.c

@@ -108,7 +108,7 @@ static void wakeup_cpu_for_thread(thread_t *t)
         mp_reschedule(1U << pinned_cpu, 0);
 }
 
-void init_thread_struct(thread_t *t, const char *name) {
+static void init_thread_struct(thread_t *t, const char *name) {
     memset(t, 0, sizeof(thread_t));
     t->magic = THREAD_MAGIC;
     thread_set_pinned_cpu(t, -1);

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;