avfilter/vf_deshake: use a void * comparator for consistency

For generality, qsort uses a comparator whose elements are void *. This
makes the comparator have such a form, and thus makes the void * cast of
the comparator pointer useless. Furthermore, this makes the code more
consistent with other usages of qsort across the codebase.

Reviewed-by: Henrik Gramner <henrik@gramner.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>