cfg80211: fix compilation warning for cfg80211_leave_all()

The following compilation issue popped up moving from v3.10-rc1 to
v3.10-rc6 after merging wireless-testing.

net/wireless/sysfs.c:86:13: error: 'cfg80211_leave_all' defined
but not used [-Werror=unused-function]

The function is only called when CONFIG_PM is enabled. Moving the
function under CONFIG_PM as well.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>