Add an int parameter to the timer_fired function

[Lzeyuan]

I encountered an error when compiling a project using make on Fedora 42, as follows:

multithreading.c:85:29: error: passing argument 2 of ‘signal’ from incompatible pointer type [-Wincompatible-pointer-types]
   85 |         if (signal(SIGALRM, timer_fired) == SIG_ERR) {
      |                             ^~~~~~~~~~~
      |                             |
      |                             void (*)(void)

The prototype of the signal function is sighandler_t signal(int signum, sighandler_t handler);
And typedef typeof(void (int)) *sighandler_t;

Compiling normally under Ubuntu 24. I think the reason under Ubuntu may be due to the loose warning policy - but this does not mean the behavior is correct, so I added an int parameter.

[kalvdans]

Could mark it static as well since it is internal to this file.