Fix DHCP unit test after my last changes

src/include/lwip/dhcp.h

@@ -120,6 +120,8 @@ void dhcp_coarse_tmr(void);
 void dhcp_fine_tmr(void);
 void dhcp_init(void);
 
+/* for use in unit tests only */
+extern u8_t dhcp_netif_client_id;
 
 #if LWIP_DHCP_GET_NTP_SRV
 /** This function must exist, in other to add offered NTP servers to