esp_netif: added missing C++ guards

Closes https://github.com/espressif/esp-idf/issues/4389

components/esp_netif/include/esp_netif_net_stack.h

@@ -15,6 +15,10 @@
 #ifndef _ESP_NETIF_NET_STACK_H_
 #define _ESP_NETIF_NET_STACK_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 //
 // Network stack API: This ESP-NETIF API are supposed to be called only from internals of TCP/IP stack
 //
@@ -76,4 +80,8 @@ void esp_netif_free_rx_buffer(void *esp_netif, void* buffer);
  * @}
  */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif //_ESP_NETIF_NET_STACK_H_