Wrap shared windows-nat code in windows_nat namespace

Wrap shared windows-nat code in windows_nat namespace

This wraps the shared windows-nat code in a windows_nat namespace.
This helps avoid name clashes.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c: Add "using namespace".
	* nat/windows-nat.h: Wrap contents in windows_nat namespace.
	* nat/windows-nat.c: Wrap contents in windows_nat namespace.

gdbserver/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* win32-low.h (struct win32_target_ops): Use qualified names where
	needed.
	* win32-i386-low.c: Add "using namespace".
	* win32-low.c: Add "using namespace".
	* win32-arm-low.c: Add "using namespace".