schema CHANGE always store extension's parent statement

So far the parent statement of extension instances was stored only in
case the extension was not stored directly in the extension list of
parent structure. But there is no reason to distinguish these cases, so
the patch makes libyang to store the extension's parent statement type
always.
diff --git a/src/tree_schema.h b/src/tree_schema.h
index cf825d1..142a33d 100644
--- a/src/tree_schema.h
+++ b/src/tree_schema.h
@@ -442,6 +442,13 @@
 const char *lyext_parent2str(LYEXT_PARENT type);
 
 /**
+ * @brief Convert nodetype to statement identifier
+ * @param[in] nodetype Nodetype to convert.
+ * @return Statement identifier representing the given @p nodetype.
+ */
+enum ly_stmt lys_nodetype2stmt(uint16_t nodetype);
+
+/**
  * @brief YANG import-stmt
  */
 struct lysp_import {