fix the following 'make includecheck' warning:

  drivers/xen/evtchn.c: linux/errno.h is included more than once.
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc:  chrisw@sous-sol.org
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <1247067749.4382.90.camel@ht.satnam>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>