xz: Silence a warning when sig_atomic_t is long int.

It can be true at least on z/OS.

src/xz/signals.c

@@ -152,7 +152,7 @@
 extern void
 signals_exit(void)
 {
-	const int sig = exit_signal;
+	const int sig = (int)exit_signal;
 
 	if (sig != 0) {
 #if defined(TUKLIB_DOSLIKE) || defined(__VMS)