Use private mapping from provided shm_pool buffer

[Caellian]

Using MAP_SHARED causes a crash on some compositors/systems as shown by #1824. According to WL docs:

From version 7 onwards, the fd must be mapped with MAP_PRIVATE by the recipient, as MAP_SHARED may fail.

We don't need a MAP_SHARED as the provided buffer will be used exclusively by conky and not shared with other processes.

This PR addresses that and closes #1824.

Testing

• Tested private mapping in another Wayland client and it worked with hyprland.

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