nlohmannjson: use full header where needed

These headers end up using non-reference, non-pointer mentions of the
nlohmannjson types, so they need the full header, not the forward decl
header.

Common/Core/vtkMarshalContext.h

@@ -23,8 +23,8 @@
 #include "vtkWeakPointer.h"      // for vtkWeakPointer
 
 // clang-format off
-#include "vtk_nlohmannjson.h"            // for json
-#include VTK_NLOHMANN_JSON(json_fwd.hpp) // for json
+#include "vtk_nlohmannjson.h"        // for json
+#include VTK_NLOHMANN_JSON(json.hpp) // for json
 // clang-format on
 
 #include <memory> // for unique_ptr

Common/Core/vtkSerializer.h

@@ -15,7 +15,7 @@
 
 // clang-format off
 #include "vtk_nlohmannjson.h"        // for json
-#include VTK_NLOHMANN_JSON(json_fwd.hpp) // for json
+#include VTK_NLOHMANN_JSON(json.hpp) // for json
 // clang-format on
 
 #include <memory>   // for unique_ptr

Rendering/Core/vtkCompositeDataDisplayAttributes.h

@@ -28,7 +28,7 @@
 
 // clang-format off
 #include "vtk_nlohmannjson.h"
-#include VTK_NLOHMANN_JSON(json_fwd.hpp)
+#include VTK_NLOHMANN_JSON(json.hpp)
 // clang-format on
 
 VTK_ABI_NAMESPACE_BEGIN