kernel: object: rename z_object_init to k_object_init

Do not use z_ for internal API and rename to k_object_init.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/stack.c b/kernel/stack.c
index 0362ece..dc6d1a6 100644
--- a/kernel/stack.c
+++ b/kernel/stack.c
@@ -32,7 +32,7 @@
 	stack->top = stack->base + num_entries;
 
 	SYS_PORT_TRACING_OBJ_INIT(k_stack, stack);
-	z_object_init(stack);
+	k_object_init(stack);
 
 #ifdef CONFIG_OBJ_CORE_STACK
 	k_obj_core_init_and_link(K_OBJ_CORE(stack), &obj_type_stack);