Merge branch 'feature/add_sys_select_header_file' into 'master'

Add <sys/select.h> header file

See merge request sdk/ESP8266_RTOS_SDK!814

components/lwip/port/esp8266/include/sys/select.h

@@ -0,0 +1,20 @@
+// Copyright 2018 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef __ESP_SYS_SELECT_H__
+#define __ESP_SYS_SELECT_H__
+
+#include "lwip/sockets.h"
+
+#endif //__ESP_SYS_SELECT_H__