libyang MAINTENANCE cleanup header includes

Use iwyu tool and fix which headers are included
diff --git a/src/validation.h b/src/validation.h
index c255270..5264cc7 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -18,8 +18,8 @@
 #include <stdint.h>
 
 #include "log.h"
-#include "parser_data.h"
 
+struct ly_ctx;
 struct ly_set;
 struct lyd_node;
 struct lys_module;