staging: unisys: remove reference of visorutil

commit 53490b54 ("staging: unisys: move periodic_work.c into the visorbus directory")
has removed the visorutil directory but missed removing the reference in
the Makefile.

Fixes: 53490b54 ("staging: unisys: move periodic_work.c into the visorbus directory")
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>