hurd: fix build of tst-system.c

We made tst-system.c depend on pthread, but that requires linking with
$(shared-thread-library). It does not fail under Linux because the
variable expands to nothing under Linux, but it fails for Hurd.

I tested verified via cross-compiling that "make check" now works
for Hurd.

Signed-off-by: Adam Yi <ayi@janestreet.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>