xen: Fix config option reference in XEN_PRIVCMD definition

Commit 2fae6bb7 ("xen/privcmd: Add new syscall to get gsi from dev")
adds a weak reverse dependency to the config XEN_PRIVCMD definition,
referring to CONFIG_XEN_PCIDEV_BACKEND. In Kconfig files, one refers to
config options without the CONFIG prefix, though. So in its current form,
this does not create the reverse dependency as intended, but is an
attribute with no effect.

Refer to the intended config option XEN_PCIDEV_BACKEND in the XEN_PRIVCMD
definition.

Fixes: 2fae6bb7 ("xen/privcmd: Add new syscall to get gsi from dev")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240930090650.429813-1-lukas.bulwahn@redhat.com>
Signed-off-by: Juergen Gross <jgross@suse.com>