structure FEATURE support for ietf-yang-structure-ext

Only for the structure itself, no augment yet.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 50783a2..faad765 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -151,6 +151,7 @@
     src/plugins_exts/nacm.c
     src/plugins_exts/yangdata.c
     src/plugins_exts/schema_mount.c
+    src/plugins_exts/structure.c
     src/xml.c
     src/xpath.c
     src/validation.c