wireless: Kconfig: add missing dependency

Previous driver changes to airo, atmel, wl3501_cs, and usb_zd1201 need
to include <net/cfg80211.h>, which depends on CFG80211, so add the
missing dependency.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>