staging: xgifb: eliminate filter_tb global variable

filter_tb is only used inside a single function, and it does not need
to be static.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>