libyang MAINTENANCE cleanup header includes

Use iwyu tool and fix which headers are included
diff --git a/src/parser_lyb.c b/src/parser_lyb.c
index b4abac3..9954377 100644
--- a/src/parser_lyb.c
+++ b/src/parser_lyb.c
@@ -30,6 +30,7 @@
 #include "log.h"
 #include "parser_data.h"
 #include "parser_internal.h"
+#include "set.h"
 #include "tree.h"
 #include "tree_data.h"
 #include "tree_data_internal.h"