win32: Fix socket fd type.

src/win32/win32_socket.c

@@ -70,7 +70,7 @@ int win32_poll(struct pollfd *fds, unsigned int nfds, int timo) {
 
     if (rc > 0) {
         for ( i = 0; i < nfds; ++i) {
-            int fd = fds[i].fd;
+            SOCKET fd = fds[i].fd;
 
             if (fds[i].events & (POLLIN | POLLPRI) && FD_ISSET(fd, &ifds)) {
                 fds[i].revents |= POLLIN;