Bug #1001857: socketmodule does not build under cygwin

Restore clean building under Cygwin.
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index f06e253..8e1bc19 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -286,6 +286,10 @@
 #include "addrinfo.h"
 #endif
 
+#if defined(__CYGWIN__)
+#include "addrinfo.h"
+#endif
+
 #ifndef HAVE_INET_PTON
 int inet_pton(int af, const char *src, void *dst);
 const char *inet_ntop(int af, const void *src, char *dst, socklen_t size);