[FYI] Fix gdb mingw build

[Joel Brobecker]

> I noticed that the mingw build was failing in the buildbot.  This
> patch fixes the problem.  I'm checking it in as obvious.
> 
> gdb/ChangeLog
> 2018-06-08  Tom Tromey  <tom@tromey.com>
> 
> 	* windows-nat.c (windows_nat_target::xfer_partial): "beneath" is
> 	now a method.

Gasp. I think this was because of me; mea culpa, and thanks a lot
for fixing, Tom.

-- 
Joel