esp_netif/tcpip_adapter: declare functions extern "C"

Merges https://github.com/espressif/esp-idf/pull/4408

components/tcpip_adapter/include/tcpip_adapter.h

@@ -23,6 +23,10 @@
 
 #include "tcpip_adapter_types.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @brief tcpip adapter legacy init. It is used only to set the compatibility mode of esp-netif, which
  * will enable backward compatibility of esp-netif.
@@ -245,4 +249,8 @@ esp_err_t tcpip_adapter_get_hostname(tcpip_adapter_if_t tcpip_if, const char **h
  */
 esp_err_t tcpip_adapter_set_default_wifi_handlers(void);
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
 #endif //_TCPIP_ADAPTER_H_