tests modem/backends/tty: Define _XOPEN_SOURCE

This test uses functions which are extensions to
the C library. Let's explicity select one of the extensions
which includes it instead of relaying on somebody having
set it for this file somewhere else.
(Note this test is exclusive to native targets)

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>