hw/xen/passthrough: add missing error-report include

hw/xen/passthrough: add missing error-report include

In commit cfcacbab38e ("xen/passthrough: use gsi to map pirq when
dom0 is PVH") an `error_report` was added to this file, but the
corresponding include of `qemu/error-report.h` was missed. This
only becomes apparent when building against Xen 4.20+ with trace
backend log disabled.

Fixes: cfcacbab38e4 (xen/passthrough: use gsi to map pirq when dom0 is PVH)
Signed-off-by: Adam Williamson <awilliam@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250717220207.171040-1-awilliam@redhat.com>
[PMD: Improved commit description, added Fixes: tag]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>