lowpan6_ble: Fix build warning if LWIP_RFC7668_LINUX_WORKAROUND_PUBLIC_ADDRESS==0

Fix unused parameter ‘public_addr’ build warning if
LWIP_RFC7668_LINUX_WORKAROUND_PUBLIC_ADDRESS==0.

Signed-off-by: Axel Lin <axel.lin@ingics.com>

src/netif/lowpan6_ble.c

@@ -116,6 +116,8 @@ void ble_addr_to_eui64(uint8_t *dst, uint8_t *src, uint8_t public_addr)
   } else {
     dst[0] |= 0x02;
   }
+#else
+  LWIP_UNUSED_ARG(public_addr);
 #endif
 }