add a function to change the type of a hf_field

Pick up change from Samba:

    commit 755bc4ed29c89fc15d4230ce8b828b3afcfa2912
    Author: Matthieu Patou <mat@matws.net>
    Date:   Thu Oct 3 01:10:48 2013 -0700

    pidl-wireshark: add a function to change the type of a hf_field

    We can use this to change the type FT_BYTE when we realize that the
    object dissected is an array of bytes

Change-Id: I1f24ce834b6a5be0d933b1fdfd9a0baa88c3761a
Reviewed-on: https://code.wireshark.org/review/5093
Reviewed-by: Guy Harris <guy@alum.mit.edu>