staging/xgifb: Include sis initdef.h header

This patch includes the initdef.h header from the sis driver.
Since the xgi driver used to redefine a lot of stuff from the sis
driver, we can simply include the headers of the sis driver itself, so
we can remove duplicated stuff later on.

In order to include the initdef.h we have to rename the header guards.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>