Staging: rtl8192u: Add select WEXT_PRIV to Kconfig to prevent build failure

Without WEXT_PRIV set the driver 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>