tests/c_lib: Set _GNU_SOURCE for qsort_r test

qsort_r is a GNU addition, so we need to #define _GNU_SOURCE to ensure
the prototype is defined in the header file.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

tests/lib/c_lib/src/test_qsort.c

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#define _GNU_SOURCE
 #include <stdbool.h>
 #include <stdlib.h>
 #include <ztest.h>