message: Fix exports of ospray_json for Windows
diff: OSPRAY_CREATE_LIBRARY(ospray_tfn tfn_lib.cpp LINK ospray_common)
  OSPRAY_CREATE_LIBRARY(ospray_json jsoncpp.cpp LINK ospray_common)
  +SET_TARGET_PROPERTIES(ospray_json PROPERTIES DEFINE_SYMBOL JSON_DLL_BUILD)
  OPTION(OSPRAY_APPS_PARAVIEW_TFN_CVT
  "Build ParaView to OSPRay viewer transfer function converter"
  