COMP: Add && to unique_ptr parameter declaration, to work around SWIG

Aims to avoid errors from ITK.Linux.Python and ITK.macOS.Python like those from
https://open.cdash.org/viewBuildError.php?buildid=9511958:

> Wrapping/Modules/ITKImageGradient/itkGradientImageFilterPython.cpp:3856:70:
> error: call to implicitly-deleted copy constructor of
> `std::unique_ptr<itk::ImageBoundaryCondition<itk::Image<float, 2>>>`

Using ITK's current SWIG revision, 2024-03-26-master.

Author: N-Dekker

Modules/Filtering/ImageGradient/include/itkGradientImageFilter.h

@@ -147,7 +147,7 @@ class ITK_TEMPLATE_EXPORT GradientImageFilter : public ImageToImageFilter<TInput
 
   /** Allows to change the default boundary condition */
   void
-  OverrideBoundaryCondition(std::unique_ptr<BoundaryConditionType> boundaryCondition);
+  OverrideBoundaryCondition(std::unique_ptr<BoundaryConditionType> && boundaryCondition);
 
 #ifdef ITK_USE_CONCEPT_CHECKING
   // Begin concept checking

Modules/Filtering/ImageGradient/include/itkGradientImageFilter.hxx

@@ -48,7 +48,7 @@ GradientImageFilter<TInputImage, TOperatorValueType, TOutputValue, TOutputImage>
 template <typename TInputImage, typename TOperatorValueType, typename TOutputValue, typename TOutputImage>
 void
 GradientImageFilter<TInputImage, TOperatorValueType, TOutputValue, TOutputImage>::OverrideBoundaryCondition(
-  std::unique_ptr<BoundaryConditionType> boundaryCondition)
+  std::unique_ptr<BoundaryConditionType> && boundaryCondition)
 {
   m_BoundaryCondition = std::move(boundaryCondition);
 }