system/physmem: Reduce cpu_physical_memory_sync_dirty_bitmap() scope

system/physmem: Reduce cpu_physical_memory_sync_dirty_bitmap() scope

cpu_physical_memory_sync_dirty_bitmap() is now only called within
system/physmem.c, by ramblock_sync_dirty_bitmap(). Reduce its scope
by making it internal to this file. Since it doesn't involve any CPU,
remove the 'cpu_' prefix.
Remove the now unneeded "qemu/rcu.h" and "system/memory.h" headers.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20251001175448.18933-17-philmd@linaro.org>