Fixed typo in include guard define

Found by a cool new warning in clang:

VTK/Rendering/FreeType/vtkMathTextUtilities.h:21:9: warning: '__vtkMathTextUtilities_h' is used as a header guard here, followed by #define of a different macro [-Wheader-guard]

        ^~~~~~~~~~~~~~~~~~~~~~~~
VTK/Rendering/FreeType/vtkMathTextUtilities.h:22:9: note: '__vtkMathTypeUtilities_h' is defined here; did you mean '__vtkMathTextUtilities_h'?

        ^~~~~~~~~~~~~~~~~~~~~~~~
        __vtkMathTextUtilities_h

Change-Id: I0ed048fcdfa69f6c47308987c8901eb0e56459a1
Merge topic 'vtkMathTextUtilities-include-guard' into master

fe579828 Fixed typo in include guard define