Fix possible Cygwin build problem

I noticed that nat/windows-nat.c checks __USEWIDE, but nothing sets it
there -- I forgot to copy over the definition when making this file.
This patch tries to fix the problem.  I don't have a Cygwin setup, so
I don't know whether this is sufficient, but it's probably necessary.

gdb/nat/windows-nat.c

@@ -20,6 +20,10 @@
 #include "nat/windows-nat.h"
 #include "gdbsupport/common-debug.h"
 
+#ifdef __CYGWIN__
+#define __USEWIDE
+#endif
+
 namespace windows_nat
 {