testsuite: Add missing #include on <stdlib.h>

This otherwise fails on newer Clangs, which warn
more aggressively on undeclared symbols.

testsuite/tests/concurrent/should_run/conc059_c.c

 #include "HsFFI.h"
 #include "conc059_stub.h"
 #include <unistd.h>
+#include <stdlib.h>
 #include <stdio.h>
 #if mingw32_HOST_OS
 #include <windows.h>