Fix Cygwin compilation after target_ops C++ conversion.

Fix Cygwin compilation after target_ops C++ conversion.

After f6ac5f3d "Convert struct target_ops to C++", we need to explicitly use
the global namespace when calling ::close() from windows_nat_target methods,
as that object has a close() method.

gdb/ChangeLog:

2018-07-14  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-nat.c (windows_nat_target::create_inferior): Update to
	call close() in global namespace.