kernel: don't use typedef for enums or non-opaque structs

arch/x86_64/pit.c

@@ -11,7 +11,7 @@ static int pit_callback(void)
 	return 0;
 }
 
-static irq_hook_t pit_irq_hook = {
+static struct irq_hook pit_irq_hook = {
 	.irq_callback = pit_callback
 };