Centralize FixMath dependency for better portability and stricter build systems

[NoNamedCat]

This set of changes addresses build issues encountered when using Mozzi with Arduino CLI and strictly structured 3rd party cores (specifically for the CH32 architecture).

The Problem:
In the original codebase, files like 'Oscil.h' and 'Line.h' included both 'mozzi_fixmath.h' (Mozzi's math abstraction) and 'FixMath.h' (the external library) explicitly. On some platforms/build systems, if the include paths are not resolved in a specific order, or if a user includes a Mozzi header that relies on FixMath types without including FixMath.h themselves, compilation fails with "unknown type" errors.

The Solution:
Moved the #include <FixMath.h> directive inside mozzi_fixmath.h.

Consequently, removed the explicit #include <FixMath.h> from:

• AudioOutput.h
• Line.h
• MetaOscil.h
• Oscil.h

Why this is safe and better:

1. Encapsulation: mozzi_fixmath.h acts as the interface for fixed-point math in Mozzi. It effectively "owns" the relationship with the underlying FixMath library. Consuming files shouldn't need to manually import the backend library.
2. Robustness: Any file including mozzi_fixmath.h is now guaranteed to have the necessary FixMath types defined, preventing subtle ordering bugs.
3. Compatibility: Since FixMath.h uses standard include guards, this change does not negatively impact existing platforms (AVR, ESP, STM32, etc.), but it fixes the build path issues on newer/stricter platforms.