Staging: wlan-ng: Add select WEXT_PRIV to Kconfig to prevent build failure

Without WEXT_PRIV set the p80211wext.c fails to build due to unknown fields in
the iw_handler_def struct.
Those fields are enclosed in WEXT_PRIV conditionals in the prototype
of iw_handler_def in include/net/iw_handler.h

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Acked-by: Simon Horman <horms@verge.net.au>
Cc: stable <stable@kernel.org>  [2.6.33 only]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>