signal-util: fix incorrect argument of va_start

The last argument of the function before the vargs is "old" not "how".

warning: second parameter of ‘va_start’ not last named argument

src/basic/signal-util.c

@@ -167,7 +167,7 @@ int sigprocmask_many(int how, sigset_t *old, ...) {
         if (sigemptyset(&ss) < 0)
                 return -errno;
 
-        va_start(ap, how);
+        va_start(ap, old);
         r = sigset_add_many_ap(&ss, ap);
         va_end(ap);