Put back #include <windows.h> in dirmod.c.
I removed this in commit
9e3755ecb, 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.