staging: unisys: visorbus: add missing include

this removes lots of sparse warnings :
warning: symbol 'visorchannel_destroy' was not declared. Should it be static?
warning: symbol 'visorchannel_get_physaddr' was not declared. Should it be static?
warning: symbol 'visorchannel_get_nbytes' was not declared. Should it be static?
warning: symbol 'visorchannel_uuid_id' was not declared. Should it be static?
warning: symbol 'visorchannel_id' was not declared. Should it be static?
warning: symbol 'visorchannel_zoneid' was not declared. Should it be static?
warning: symbol 'visorchannel_get_clientpartition' was not declared. Should it be static?
warning: symbol 'visorchannel_set_clientpartition' was not declared. Should it be static?
warning: symbol 'visorchannel_read' was not declared. Should it be static?
warning: symbol 'visorchannel_write' was not declared. Should it be static?
warning: symbol 'visorchannel_get_header' was not declared. Should it be static?
warning: symbol 'visorchannel_create' was not declared. Should it be static?
warning: symbol 'visorchannel_create_with_lock' was not declared. Should it be static?

Signed-off-by: Laurent Navet <laurent.navet@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>