rtlwifi: include mac80211.h from core.h

core.h references ieee80211_ops but doesn't include mac80211.h, which issues
a warning once mac80211-compat.h is taken into use.

Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>