c/syscalls.c: fix gethostname() prototype

[wishstudio]

Fix the prototype of gethostname() function to fix gcc 15 build error:

c/syscalls.c: In function 'scm_gethostname':
c/syscalls.c:673:9: error: conflicting types for 'gethostname'; have 'int(void)'
  673 |     int gethostname();
      |         ^~~~~~~~~~~
In file included from /nix/store/fbbw928argckfii0j322346ihmllg7a7-glibc-2.42-61-dev/include/features.h:524,
                 from /nix/store/fbbw928argckfii0j322346ihmllg7a7-glibc-2.42-61-dev/include/bits/libc-header-start.h:33,
                 from /nix/store/fbbw928argckfii0j322346ihmllg7a7-glibc-2.42-61-dev/include/stdio.h:28,
                 from c/syscalls.c:6:
/nix/store/fbbw928argckfii0j322346ihmllg7a7-glibc-2.42-61-dev/include/bits/unistd.h:189:1: note: previous definition of 'gethostname' with type 'int(char *, long unsigned int)'
  189 | __NTH (gethostname (__fortify_clang_overload_arg (char *, ,__buf),
      | ^~~~~
c/syscalls.c:674:18: error: too many arguments to function 'gethostname'; expected 0, have 2
  674 |     int retval = gethostname(hostname, MAXHOSTNAMELEN);
      |                  ^~~~~~~~~~~ ~~~~~~~~
c/syscalls.c:673:9: note: declared here
  673 |     int gethostname();
      |         ^~~~~~~~~~~