mac80211: fix warning for un-used parameter

mesh_hdr only used when CONFIG_MAC80211_MESH is defined
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -1701,7 +1701,7 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 	u16 ethertype, hdrlen,  meshhdrlen = 0;
 	__le16 fc;
 	struct ieee80211_hdr hdr;
-	struct ieee80211s_hdr mesh_hdr;
+	struct ieee80211s_hdr mesh_hdr __maybe_unused;
 	const u8 *encaps_data;
 	int encaps_len, skip_header_bytes;
 	int nh_pos, h_pos;