Only one buffer.c, please.

Otherwise, if you link with both libwiretap and libfiletap, it's
anybody's guess which one you get.  That means you're wasting memory
with two copies of its routines if they're identical, and means
surprising behavior if they're not (which showed up when I was debugging
a double-free crash - fixing libwiretap's buffer_free() didn't fix the
problem, because Wireshark happened to be calling libfiletap' unfixed
buffer_free()).

There's nothing *tap-specific about Buffers, anyway, so it really
belongs in wsutil.

Change-Id: I91537e46917e91277981f8f3365a2c0873152870
Reviewed-on: https://code.wireshark.org/review/3066
Reviewed-by: Guy Harris <guy@alum.mit.edu>