[binutils-gdb] Fix gdb mingw build

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 2f4f025ff15b900706c8597060003a506107cf23 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 2f4f025ff15b900706c8597060003a506107cf23

Fix gdb mingw build

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.