accel: Forward-declare AccelOpsClass in 'qemu/typedefs.h'

accel: Forward-declare AccelOpsClass in 'qemu/typedefs.h'

The heavily imported "system/cpus.h" header includes "accel-ops.h"
to get AccelOpsClass type declaration. Reduce headers pressure by
forward declaring it in "qemu/typedefs.h", where we already
declare the AccelCPUState type.

Reduce "system/cpus.h" inclusions by only including
"system/accel-ops.h" when necessary.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250123234415.59850-14-philmd@linaro.org>