Cygwin: lseek: return ESPIPE rather than EINVAL when called on a fifo

Thanks to Henri for tracking this down:
https://cygwin.com/ml/cygwin/2018-10/msg00062.html

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -1237,6 +1237,7 @@ class fhandler_fifo: public fhandler_base_overlapped
 public:
   fhandler_fifo ();
   int open (int, mode_t);
+  off_t lseek (off_t offset, int whence);
   int close ();
   int dup (fhandler_base *child, int);
   bool isfifo () const { return true; }