Staging: xgifb: remove externs in XGI_accel.c

Remove 2 extern variables definations from XGI_accel.c that were
defined in other .h files.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Cc: Arnaud Patard <apatard@mandriva.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>