esp_netif: added missing C++ guards

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

components/esp_netif/include/esp_netif_ip_addr.h

@@ -16,6 +16,11 @@
 #define _ESP_NETIF_IP_ADDR_H_
 
 #include <machine/endian.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if BYTE_ORDER == BIG_ENDIAN
 #define esp_netif_htonl(x) ((uint32_t)(x))
 #else
@@ -95,4 +100,8 @@ typedef struct _ip_addr {
     uint8_t type;
 } esp_ip_addr_t;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif //_ESP_NETIF_IP_ADDR_H_