/usr/include/sbml/extension/ is in libsbml5-dev 5.12.0+dfsg-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/sbml/extension/ASTBasePlugin.h
/usr/include/sbml/extension/ISBMLExtensionNamespaces.h
/usr/include/sbml/extension/RegisterExtensions.h
/usr/include/sbml/extension/SBMLDocumentPlugin.h
/usr/include/sbml/extension/SBMLDocumentPluginNotRequired.h
/usr/include/sbml/extension/SBMLExtension.h
/usr/include/sbml/extension/SBMLExtensionException.h
/usr/include/sbml/extension/SBMLExtensionNamespaces.h
/usr/include/sbml/extension/SBMLExtensionRegister.h
/usr/include/sbml/extension/SBMLExtensionRegistry.h
/usr/include/sbml/extension/SBaseExtensionPoint.h
/usr/include/sbml/extension/SBasePlugin.h
/usr/include/sbml/extension/SBasePluginCreator.h
/usr/include/sbml/extension/SBasePluginCreatorBase.h