headers MAINTENANCE headers/include cleanup

- add missing extern "C" in some of the public headers for use in C++
- includes cleanup and fix ordering
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9db7241..7799520 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -250,21 +250,19 @@
 set(headers
     src/libyang.h
     src/context.h
-    src/tree.h
-    src/tree_data.h
+    src/dict.h
+    src/log.h
     src/parser.h
     src/parser_schema.h
     src/plugins_exts.h
     src/plugins_types.h
     src/printer.h
     src/printer_data.h
-    src/tree_schema.h
     src/printer_schema.h
-    src/plugins_exts.h
-    src/plugins_types.h
-    src/dict.h
-    src/log.h
-    src/set.h)
+    src/set.h
+    src/tree.h
+    src/tree_data.h
+    src/tree_schema.h)
 
 # create static libyang library
 if(ENABLE_STATIC)