fixed a warning (int->long int)

Signed-off-by: Steven Honeyman <stevenhoneyman@users.noreply.github.com>

x.cpp

     char buffer[1024];
     XGetErrorText( dpy, event->error_code, buffer, 1024 );
     fprintf( stderr,
-             "_X Error of failed request:  %s\n_  Major opcode of failed request: % 3d\n_  Serial number of failed request:% 5d\n_  Current serial number in output stream:?????\n",
+             "_X Error of failed request:  %s\n_  Major opcode of failed request: % 3d\n_  Serial number of failed request:% 5li\n_  Current serial number in output stream:?????\n",
              buffer,
              event->request_code,
              event->serial );