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/src/parser_yang.c b/src/parser_yang.c
index df78866..93231a8 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -25,6 +25,7 @@
 #include "dict.h"
 #include "log.h"
 #include "path.h"
+#include "parser_schema.h"
 #include "set.h"
 #include "tree.h"
 #include "tree_schema.h"