SystemCoreClock include header

[KnockbackNemo]

Describe the problem this feature will solve
In order to initialize the OS_CPU_SysTickInit, we need SystemCoreClock, but this isn't in Tasks.h or common.h.

Describe the proposed solution
Perhaps we'd like to change what's included?

Describe possible alternatives
Or not since it seems to generally not be a problem. I only ran into this issue when trying to write a simple test file without much else of our codebase.