Add Convex Hull in C

[rzuckerm]

To request a new code snippet, please fill out the following:

Project name: Convex Hull
Project link: https://sampleprograms.io/projects/convex-hull
Language: C

The project link must be a link to a project on the Sample Programs Website project list.
If you would like to add a new project to the Sample Programs Website,
please first make a pull request to the Sample Programs Website.

[2Clutch]

I'd like to be assigned this task if that's okay.

[rzuckerm]

@2Clutch You are assigned.