libyang CHANGE headers includes cleanup using include-what-you-use tool (#1098)

diff --git a/src/tree_schema.c b/src/tree_schema.c
index 958c342..d7b7da7 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -12,6 +12,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
+#define _GNU_SOURCE
+
 #include "common.h"
 
 #include <assert.h>
@@ -31,6 +33,7 @@
 #include "log.h"
 #include "set.h"
 #include "xpath.h"
+#include "plugins_exts.h"
 #include "tree.h"
 #include "tree_schema.h"
 #include "tree_schema_internal.h"