wireless: mark cfg80211_is_all_idle as static

  CHECK   net/wireless/sme.c
net/wireless/sme.c:38:6: warning: symbol 'cfg80211_is_all_idle' was not declared. Should it be static?

It is not used elsewhere, so mark it static.
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/wireless/sme.c

@@ -35,7 +35,7 @@ struct cfg80211_conn {
 	bool auto_auth, prev_bssid_valid;
 };
 
-bool cfg80211_is_all_idle(void)
+static bool cfg80211_is_all_idle(void)
 {
 	struct cfg80211_registered_device *rdev;
 	struct wireless_dev *wdev;