libertas: select WIRELESS_EXT

Ensure that libertas selects WIRELESS_EXT, since selecting other stuff that
should depend on WEXT, like IEEE80211, doesn't seem to drag that in for us.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>