extensions CHANGE compile internal plugins separately

Avoid including *.c files into other c file and let them compile
individualy. The change needs also to correct includes in the plugins
source codes.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c081e35..3f8d53c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -135,6 +135,9 @@
     src/printer_tree.c
     src/plugins_types.c
     src/plugins_exts.c
+    src/plugins_exts_metadata.c
+    src/plugins_exts_nacm.c
+    src/plugins_exts_yangdata.c
     src/xml.c
     src/xpath.c
     src/validation.c)