Fix warning in nptl/tst-stack4.c.

Fix warning in nptl/tst-stack4.c.

This patch fixes a warning in a test that was added since my recent
warning cleanup:

tst-stack4.c: In function 'dso_process':
tst-stack4.c:64:7: warning: format '%i' expects argument of type 'int', but argument 3 has type 'uintptr_t' [-Wformat=]

The original variable has type int then is cast to uintptr_t, and from
there to void *, to pass it to a thread, so reversing the process by
casting to uintptr_t and then to int is natural; this patch does so.

Tested for x86_64.

	* nptl/tst-stack4.c (dso_process): Use int not uintptr_t for t.