COMP: class/struct consistency between forward declaration and definition

- silence clang warnings

src/OSspecific/POSIX/signals/sigStopAtWriteNow.C

@@ -53,10 +53,12 @@ static Foam::Time const* runTimePtr_ = nullptr;
 namespace Foam
 {
 // Register re-reader
-struct addstopAtWriteNowSignalToOpt
+class addstopAtWriteNowSignalToOpt
 :
     public ::Foam::simpleRegIOobject
 {
+public:
+
     addstopAtWriteNowSignalToOpt(const char* name)
     :
         ::Foam::simpleRegIOobject(Foam::debug::addOptimisationObject, name)

src/OSspecific/POSIX/signals/sigWriteNow.C

@@ -53,10 +53,12 @@ namespace Foam
 {
 
 // Register re-reader
-struct addwriteNowSignalToOpt
+class addwriteNowSignalToOpt
 :
     public ::Foam::simpleRegIOobject
 {
+public:
+
     addwriteNowSignalToOpt(const char* name)
     :
         ::Foam::simpleRegIOobject(Foam::debug::addOptimisationObject, name)

src/conversion/vtk/adaptor/foamVtkVtuAdaptor.H

@@ -74,11 +74,13 @@ namespace vtk
 \*---------------------------------------------------------------------------*/
 
 //- Bookkeeping for vtkUnstructuredGrid
-struct vtuAdaptor
+class vtuAdaptor
 :
     public vtk::Caching<vtkUnstructuredGrid>,
     public foamVtkMeshMaps
 {
+public:
+
     // Member Functions
 
     // Mesh Conversion