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/timer.c b/kernel/timer.c
index 9994ae4..1dc60db 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -128,7 +128,7 @@
 
 	timer->user_data = NULL;
 
-	z_object_init(timer);
+	k_object_init(timer);
 
 #ifdef CONFIG_OBJ_CORE_TIMER
 	k_obj_core_init_and_link(K_OBJ_CORE(timer), &obj_type_timer);