mac80211: add missing include

Signed-off-by: Felix Fietkau <nbd@nbd.name>

mac80211.c

@@ -13,6 +13,7 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
+#include <linux/of.h>
 #include "mt76.h"
 
 #define CHAN2G(_idx, _freq) {			\