Fix error: misplaced deprecation attributes in vtkVertexBufferObject{,Mapper}

[SunBlack]

Fixes an issue introduced in #4065. Didn't saw it earlier as gcc8 doesn't complain and my Clang 10 build is still broken.

/home/sunblack/dev/pcl/visualization/include/pcl/visualization/vtk/vtkVertexBufferObjectMapper.h:39:6: error: misplaced attributes; expected attributes here
class PCL_EXPORTS vtkVertexBufferObjectMapper : public vtkMapper
     ^
     PCL_DEPRECATED(1, 13, "The OpenGL backend of VTK is deprecated. Please switch to the OpenGL2 backend.")

Note: Running clang-format will be fun for this:

class PCL_DEPRECATED(
    1,
    13,
    "The OpenGL backend of VTK is deprecated. Please switch to the OpenGL2 backend.")
    PCL_EXPORTS vtkVertexBufferObject : public vtkObject {

[SergioRAgostinho]

This highlights one of the stages we forgot in the release cycle. A release candidate :)

[SunBlack]

Interesting thing: MSVC complains it like Clang 10. Sad GCC doesn't :-(