selftest: fix build error in some configurations

Include stdlib.h for EXIT_FAILURE.
diff --git a/programs/test/selftest.c b/programs/test/selftest.c
index d21d47c..846d34d 100644
--- a/programs/test/selftest.c
+++ b/programs/test/selftest.c
@@ -51,6 +51,7 @@
 #include "mbedtls/ecp.h"
 #include "mbedtls/timing.h"
 
+#include <stdlib.h>
 #include <stdio.h>
 #include <string.h>