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

diff --git a/tests/utests/test_hash_table.c b/tests/utests/test_hash_table.c
index adececa..f705db0 100644
--- a/tests/utests/test_hash_table.c
+++ b/tests/utests/test_hash_table.c
@@ -12,6 +12,8 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
+#define _GNU_SOURCE
+
 #include "common.h"
 
 #include "tests/config.h"