Put back #include <windows.h> in dirmod.c.

I removed this in commit 9e3755ec, reasoning that the win32.h
port-specific header file included by c.h would have provided it.
However, that's only true on native win32 builds, not Cygwin builds.

It may be that some of the other <windows.h> inclusions also need
to be put back on the same grounds; but this is the only one that
is clearly meant to be included #ifdef __CYGWIN__, so maybe this is
the extent of the problem.  Awaiting further buildfarm results.

src/port/dirmod.c

@@ -34,6 +34,7 @@
 #ifndef __CYGWIN__
 #include <winioctl.h>
 #else
+#include <windows.h>
 #include <w32api/winioctl.h>
 #endif
 #endif