ath11k: Fix build warning without CONFIG_IPV6

drivers/net/wireless/ath/ath11k/mac.c:8175:13: error: ‘ath11k_mac_op_ipv6_changed’ defined but not used [-Werror=unused-function]
 static void ath11k_mac_op_ipv6_changed(struct ieee80211_hw *hw,
             ^~~~~~~~~~~~~~~~~~~~~~~~~~

Wrap it with #ifdef block to fix this.

Fixes: c3c36bfe ("ath11k: support ARP and NS offload")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220411020843.10284-1-yuehaibing@huawei.com

drivers/net/wireless/ath/ath11k/mac.c

@@ -8145,6 +8145,7 @@ static void ath11k_mac_op_sta_statistics(struct ieee80211_hw *hw,
 	}
 }
 
+#if IS_ENABLED(CONFIG_IPV6)
 static void ath11k_generate_ns_mc_addr(struct ath11k *ar,
 				       struct ath11k_arp_ns_offload *offload)
 {
@@ -8239,6 +8240,7 @@ static void ath11k_mac_op_ipv6_changed(struct ieee80211_hw *hw,
 	/* generate ns multicast address */
 	ath11k_generate_ns_mc_addr(ar, offload);
 }
+#endif
 
 static void ath11k_mac_op_set_rekey_data(struct ieee80211_hw *hw,
 					 struct ieee80211_vif *vif,