cfg80211: don't allow disabling WEXT if it's required

The change to only export WEXT symbols when required could break
the build if CONFIG_CFG80211_WEXT was explicitly disabled while
a driver like orinoco selected it.

Fix this by hiding the symbol when it's required so it can't be
disabled in that case.

Fixes: 2afe38d1 ("cfg80211-wext: export symbols only when needed")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>