Randomize pivot in quicksort [C]

Median function needs to be added in C implementation of quicksort.

- [ ] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [ ] Assign this issue to me (I want to work on this).
- [ ] I ensure that I am not already assigned to 2 or more issues.
- [ ] I will send my PR only after I'm assigned to this issue by any maintainer.
- [ ] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Locate the C implementation of quicksort and inspect how its pivot is currently selected. Check the repository’s existing algorithm conventions before deciding how the requested median or randomized pivot behavior fits; done when the C quicksort implementation includes the requested change and its relevant checks pass.

Written by the indexing model from the issue text.