libyang CHANGE unify the statement enumerations

Instead of keeping a separate enumeration for YANG extension pattern
statements, just use the standard ly_stmt enumeration.
diff --git a/src/tree_schema_helpers.c b/src/tree_schema_helpers.c
index 2f0bf43..717f2b9 100644
--- a/src/tree_schema_helpers.c
+++ b/src/tree_schema_helpers.c
@@ -1782,7 +1782,7 @@
 }
 
 LY_ARRAY_COUNT_TYPE
-lysp_ext_instance_iter(struct lysp_ext_instance *ext, LY_ARRAY_COUNT_TYPE index, LYEXT_SUBSTMT substmt)
+lysp_ext_instance_iter(struct lysp_ext_instance *ext, LY_ARRAY_COUNT_TYPE index, enum ly_stmt substmt)
 {
     LY_CHECK_ARG_RET(NULL, ext, LY_EINVAL);