COMP: Fix older gcc build error including "cstddef" for "ptrdiff_t"

This commit fixes a regression likely introduced in c173dfd (STYLE: Remove
`std::` prefix from types that work without it) to support building ITK
wheels using manylinux_2_24 providing gcc 6.3.

It fixes build error like the following:

  In file included from /work/ITK-source/ITK/Modules/Core/Common/include/itkImageRegion.h:33:0,
                   from /work/ITK-source/ITK/Modules/Core/Common/include/itkMultiThreaderBase.h:35,
                   from /work/ITK-source/ITK/Modules/Core/Common/src/itkTotalProgressReporter.cxx:19:
  /work/ITK-source/ITK/Modules/Core/Common/include/itkSize.h:242:27: error: ‘ptrdiff_t’ does not name a type
     using difference_type = ptrdiff_t;
                             ^~~~~~~~~

Modules/Compatibility/Deprecated/include/itkAtomicInt.h

@@ -39,6 +39,7 @@
 #include "itkConceptChecking.h"
 
 #include <atomic>
+#include <cstddef> // For ptrdiff_t.
 
 
 namespace itk

Modules/Core/Common/include/itkIndex.h

@@ -20,6 +20,8 @@
 
 #include "itkMakeFilled.h"
 #include "itkOffset.h"
+
+#include <cstddef>     // For ptrdiff_t.
 #include <type_traits> // For is_integral.
 
 namespace itk

Modules/Core/Common/include/itkOffset.h

@@ -21,6 +21,8 @@
 #include "itkSize.h"
 #include "itkMath.h"
 
+#include <cstddef> // For ptrdiff_t.
+
 namespace itk
 {
 

Modules/Core/Common/include/itkSize.h

@@ -22,6 +22,7 @@
 #include "itkMacro.h"
 #include "itkMakeFilled.h"
 #include <algorithm>   // For copy_n.
+#include <cstddef>     // For ptrdiff_t.
 #include <type_traits> // For is_integral.
 #include <memory>
 

Modules/IO/ImageBase/include/itkImageSeriesReader.hxx

@@ -25,6 +25,7 @@
 #include "itkMath.h"
 #include "itkProgressReporter.h"
 #include "itkMetaDataObject.h"
+#include <cstddef> // For ptrdiff_t.
 #include <iomanip>
 
 namespace itk