[vtk-developers] Merge request fixing issue with system JsonCpp on Windows.

[Hahn, Steven E.]

I encountered a linking error about undefined symbols when building
ParaView on Windows with
VTK_USE_SYSTEM_JSONCPP=ON. This error goes away and the build finishes if
JSON_DLL is defined.

This merge request (https://gitlab.kitware.com/vtk/vtk/merge_requests/994)
changes the Windows
declspec import logic so it doesn't depend on whether
VTK_USE_SYSTEM_JSONCPP is ON or OFF.


Best,
Steven Hahn