esp_netif: added missing C++ guards

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

components/esp_netif/include/esp_netif.h

@@ -22,6 +22,10 @@
 #include "esp_netif_types.h"
 #include "esp_netif_defaults.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 //
 // Note: tcpip_adapter legacy API has to be included by default to provide full compatibility
 //  for applications that used tcpip_adapter API without explicit inclusion of tcpip_adapter.h
@@ -736,4 +740,8 @@ size_t esp_netif_get_nr_of_ifs(void);
  * @}
  */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /*  _ESP_NETIF_H_ */