Move "qemu/accel.h" include from the heavily included
"hw/boards.h" to hw/core/machine.c, the single file using
the AccelState definition.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20221130135641.85328-3-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>