Fix handle_sigterm_pid1() signature

[apoikos]

This is a signal handler, so it should accept a signal number as the only argument, otherwise GCC 15 fails with the following error:

  main.c: In function 'set_sig_handlers':
  main.c:75:23: error: assignment to '__sighandler_t' {aka 'void (*)(int)'} from incompatible pointer type 'void (*)(void)' [-Wincompatible-pointer-types]
     75 |         sa.sa_handler = handle_sigterm_pid1;
        |                       ^
  main.c:40:1: note: 'handle_sigterm_pid1' declared here
     40 | handle_sigterm_pid1()
        | ^~~~~~~~~~~~~~~~~~~
  In file included from main.c:3:
  /usr/include/signal.h:72:16: note: '__sighandler_t' declared here
     72 | typedef void (*__sighandler_t) (int);
        |                ^~~~~~~~~~~~~~

See also https://bugs.debian.org/1096364.