hw/display/sm501: fix missing error-report.h

hw/display/sm501: fix missing error-report.h

"qemu/error-report.h" was previously implicitly included. This is no
longer the case following 012842c075520dbe1bd96a2fdcf4e218874ba443.

However, the issue predates this change as `error-report.h` should have
been included when the `warn_report` call was introduced.

Fixes: fa140b9562 ("hw/sm501: allow compiling without PIXMAN")
Signed-off-by: Clément Chigot <chigot@adacore.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250728090518.963573-1-chigot@adacore.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>