schema tree CHANGE base for compiling extension instances

Just a generic part, does not work with extension plugins yet.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d06d2e6..0747637 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -180,6 +180,7 @@
     src/libyang.h
     src/context.h
     src/tree_schema.h
+    src/extensions.h
     src/dict.h
     src/log.h
     src/set.h)