os: Fix build of xserver_poll.c on MinGW

Include winsock.h for definition of fd_set type and select()

Future work: Maybe this also needs some error checking for fd > FD_SETSIZE?

os/xserver_poll.c

@@ -84,6 +84,10 @@
 #include <string.h>                          /* string functions */
 #include "xserver_poll.h"
 
+#if defined(WIN32) && !defined(__CYGWIN__)
+#include <X11/Xwinsock.h>
+#endif
+
 /*---------------------------------------------------------------------------*\
 				  Macros
 \*---------------------------------------------------------------------------*/