tests: posix: headers: check for sockatmark()

Add a check for sockatmark().

Signed-off-by: Chris Friedt <chrisfriedt@gmail.com>

tests/posix/headers/src/sys_socket_h.c

@@ -115,7 +115,7 @@ ZTEST(posix_headers, test_sys_socket_h)
 		zassert_not_null(sendto);
 		zassert_not_null(setsockopt);
 		zassert_not_null(shutdown);
-		/* zassert_not_null(sockatmark); */ /* not implemented */
+		zassert_not_null(sockatmark);
 		zassert_not_null(socket);
 		zassert_not_null(socketpair);
 	}