wireshark: Switch header files to #pragma once

wireshark: Switch header files to #pragma once

The genxdrstub.pl script generates some header files. But they
use the old pattern to guard against multiple inclusion:

  #ifndef SOMETHING_H
  #define SOMETHING_H
  ...
  #endif

Change the script to generate just '#pragma once' used everywhere
else in our code.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>