gnrc_netif: add l2addr members to doc

The `l2addr` and `l2addr_len` members of the `gnrc_netif_t` struct did
not show up in the rendered documentation.

sys/include/net/gnrc/netif.h

@@ -76,7 +76,7 @@ typedef struct {
      * @see net_gnrc_netif_flags
      */
     uint32_t flags;
-#if (GNRC_NETIF_L2ADDR_MAXLEN > 0)
+#if (GNRC_NETIF_L2ADDR_MAXLEN > 0) || DOXYGEN
     /**
      * @brief   The link-layer address currently used as the source address
      *          on this interface.