hw/arm/sbsa-ref: Include missing 'sysemu/kvm.h' header

hw/arm/sbsa-ref: Include missing 'sysemu/kvm.h' header

"sysemu/kvm.h" is indirectly pulled in. Explicit its
inclusion to avoid when refactoring include/:

  hw/arm/sbsa-ref.c:693:9: error: implicit declaration of function 'kvm_enabled' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    if (kvm_enabled()) {
        ^

Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230405160454.97436-6-philmd@linaro.org>