avoid a "FILE" name conflict with cygwin's wchar.h

The 1.7.28 release of cygwin (or possible a separate package released
around the same time) header wchar.h includes the following code:

  typedef __FILE FILE;

With PERLIO_NOT_STDIO set to true, the default for core source files, we
#include nostdio.h which does:

  struct _FILE;
  #define FILE struct _FILE

which turns the above code into:

  typedef __FILE struct _FILE;

which isn't C.

Disable this hack for cygwin.c