CMake: ensure shiboken does not introduce Py_LIMITED_API.

On some builds, the CMake package files for PySide6's Shiboken set the Py_LIMITED_API define on
the compiler's command line.  This breaks FreeCAD as it depends on definitions from the Python
Stable API unavailable with the Limited API.

cMake/FreeCAD_Helpers/SetupShibokenAndPyside.cmake

@@ -37,6 +37,10 @@ macro(SetupShibokenAndPyside)
             message(STATUS "PYTHON_CONFIG_SUFFIX: ${PYTHON_CONFIG_SUFFIX}")
             find_package(Shiboken${SHIBOKEN_MAJOR_VERSION} QUIET)
         endif()
+
+        if(SHIBOKEN_MAJOR_VERSION EQUAL 6)
+            set_target_properties(Shiboken6::libshiboken PROPERTIES INTERFACE_COMPILE_DEFINITIONS "NDEBUG")
+        endif()
     endif()
 
     # pyside2 changed its cmake files, this is the dance we have