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: 2afe38d15cee ("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>

net/wireless/Kconfig

@@ -175,7 +175,7 @@ config CFG80211_INTERNAL_REGDB
 	  Most distributions have a CRDA package.  So if unsure, say N.
 
 config CFG80211_WEXT
-	bool "cfg80211 wireless extensions compatibility"
+	bool "cfg80211 wireless extensions compatibility" if !CFG80211_WEXT_EXPORT
 	depends on CFG80211
 	select WEXT_CORE
 	default y if CFG80211_WEXT_EXPORT