Mark FT_Renderer_Class::raster_class as pointer to const

The `FT_DEFINE_RASTER_FUNCS` macro declares a `const FT_Raster_Funcs`.
The address of the definition is taken and assigned to
`FT_Renderer_Class::raster_class` which is currently `FT_Raster_Funcs*`.
Until recently the `const` was cast away and with the removal of the
cast there are now warnings about dropping this `const`. Instead of
adding back the casts, make `FT_Renderer_Class::raster_class` a pointer
to const, as is done with pointers in other interfaces.

* include/freetype/ftrender.h (FT_Renderer_Class_): mark `raster_class`
as const.