rtlwifi: Fix randconfig build error with next-20140930

Configuration randconfig-1412075201 gets an error because driver rtl8821ae
depends on driver btcoexist, but Kconfig does not select it.

Reported-by: Jim Davis <jim.epost@gmail.com>
Cc: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>