Blender Git Loki

Fluid: Use hidden symbol visibility

This resolves a long list of linker warnings that is currently only showing up on macOS arm builds.
The warnings themselves are of this shape (one example):

```
ld: warning: direct access in function 'Manta::MeshDataImpl<Manta::Vector3D<float> >::_W_39(_object, object, object*)' from file '../../lib/libextern_mantaflow.a(mesh.h.reg.cpp.o)' to global weak symbol 'typeinfo for Manta::MeshDataImpl<Manta::Vector3D<float> >' from file '../../lib/libextern_mantaflow.a(mesh.cpp.o)' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.
```

Just would like to get your opinion to make sure this is an acceptable way to handle this on all platforms.

Reviewed By: LazyDodo

Differential Revision: https://developer.blender.org/D9002