Ztest: Including the missing C standard library header

The ztest.c calls atoi, which is provided by the C standard library.

Signed-off-by: Firas Sammoura <fsammoura@google.com>

subsys/testsuite/ztest/src/ztest.c

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <stdlib.h>
 #include <zephyr/ztest.h>
 
 #include <zephyr/app_memory/app_memdomain.h>