gh-148575: Use the right entry to read the set of file descriptors on CYGWIN

This patch fixes issue #148575.

Author: carlo-bramini

Modules/_posixsubprocess.c

@@ -63,7 +63,7 @@
 # endif
 #endif
 
-#if defined(__FreeBSD__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__DragonFly__)
+#if defined(__CYGWIN__) || defined(__FreeBSD__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__DragonFly__)
 # define FD_DIR "/dev/fd"
 #else
 # define FD_DIR "/proc/self/fd"