libyang MAINTENANCE cleanup header includes

Use iwyu tool and fix which headers are included
diff --git a/src/parser_stmt.c b/src/parser_stmt.c
index a560ee5..ed459de 100644
--- a/src/parser_stmt.c
+++ b/src/parser_stmt.c
@@ -21,12 +21,11 @@
 
 #include "common.h"
 #include "dict.h"
-#include "in.h"
-#include "in_internal.h"
 #include "log.h"
 #include "parser_schema.h"
 #include "path.h"
 #include "schema_compile.h"
+#include "set.h"
 #include "tree.h"
 #include "tree_edit.h"
 #include "tree_schema.h"