xen/privcmd: fix condition in privcmd_close()

The parenthesis are in the wrong place so the original code is
equivalent to:

	if (!xen_feature(XENFEAT_writable_descriptor_tables)) { ...

Which obviously was not intended.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>