staging: xgifb: add missing #include

Add a missing include to eliminate the following sparse warning:

drivers/staging/xgifb/vb_table.h:2572:28: warning: symbol 'XGI21_LCDCapList' was not declared. Should it be static?

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>