syscall: rename z_object_find -> k_object_find

Rename internal API to not use z_/Z_.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/dynamic.c b/kernel/dynamic.c
index 3fea6b6..d30dba8 100644
--- a/kernel/dynamic.c
+++ b/kernel/dynamic.c
@@ -152,7 +152,7 @@
 
 	if (IS_ENABLED(CONFIG_DYNAMIC_THREAD_ALLOC)) {
 #ifdef CONFIG_USERSPACE
-		if (z_object_find(stack)) {
+		if (k_object_find(stack)) {
 			k_object_free(stack);
 		} else {
 			k_free(stack);