Reflect the move of the buffer_ routines to libwsutil.

We went with the whole WS_DLL_EXPORT thing so that we don't *have* to
maintain lists of exported symbols; is there truly no way to automate
the generation of *these* files?

Change-Id: I77f240c77782ed634e4620833f951c4a02fb4390
Reviewed-on: https://code.wireshark.org/review/3083
Reviewed-by: Guy Harris <guy@alum.mit.edu>