Fix typo in the freetype name mangling header; issue #18478
The typo causes conflicts when linking static VTK builds with other libraries also using freetype. The following:
#define vtkfreetype_FT_MulFix vtkfreetype_vtkfreetype_FT_MulFix
needs to change to:
#define FT_MulFix vtkfreetype_FT_MulFix