sockets: guard declaration of 'lwip_select()' with LWIP_SOCKET_SELECT==1

src/include/lwip/sockets.h

@@ -480,8 +480,10 @@ int lwip_sendto(int s, const void *dataptr, size_t size, int flags,
 int lwip_socket(int domain, int type, int protocol);
 int lwip_write(int s, const void *dataptr, size_t size);
 int lwip_writev(int s, const struct iovec *iov, int iovcnt);
+#if LWIP_SOCKET_SELECT
 int lwip_select(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset,
                 struct timeval *timeout);
+#endif
 int lwip_ioctl(int s, long cmd, void *argp);
 int lwip_fcntl(int s, int cmd, int val);