dix-config.h: add HAVE_SOCKLEN_T definition

Needed to build with IPv6 disabled using gcc 14 on some platforms to avoid:

In file included from /usr/X11/include/X11/Xtrans/transport.c:67,
                 from xstrans.c:17:
/usr/X11/include/X11/Xtrans/Xtranssock.c: In function ‘_XSERVTransSocketOpen’:
/usr/X11/include/X11/Xtrans/Xtranssock.c:467:28: error: passing argument 5
 of ‘getsockopt’ from incompatible pointer type [-Wincompatible-pointer-types]
  467 |             (char *) &val, &len) == 0 && val < 64 * 1024)
      |                            ^~~~
      |                            |
      |                            size_t * {aka long unsigned int *}

Signed-off-by: Alan Coopersmith