[testsuite/darwin] fix conc059

This resolves the following:

Compile failed (exit code 1) errors were:
conc059_c.c:27:5: error:
     error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
        exit(0);
        ^
conc059_c.c:27:5: error:
     note: include the header <stdlib.h> or explicitly provide a declaration for 'exit'

testsuite/tests/concurrent/should_run/conc059_c.c

@@ -2,6 +2,8 @@
 #include "conc059_stub.h"
 #include <unistd.h>
 #include <stdio.h>
+// stdlib is needed for exit()
+#include <stdlib.h>
 #if mingw32_HOST_OS
 #include <windows.h>
 #endif