Prevent the compiler finding the wrong CGFloat

[matiasvillaverde]

As open on this Issue, this small PR allows to use the library on iOS, WatchOS and tvOS.

Checklist

• [x] The provided tests still run.
• [ ] I've created new tests where needed.
• [ ] I've updated the documentation if necessary.

Motivation and Context

Then we can use the library on iOS, the issue is this one

Description

Adding a compiler flag, to import CoreGraphics will fix the compiler getting crazy of which CGFloat to use.

[MrLotU]

Now available in https://github.com/MrLotU/SwiftPrometheus/releases/tag/1.0.0-alpha.7