[Exp PyROOT] Some headers of Cppyy need to be public

Make sure they are placed in ${build_dir}/include and
${install_dir}/include.

bindings/pyroot_experimental/cppyy/CPyCppyy/CMakeLists.txt

@@ -53,3 +53,6 @@ target_link_libraries(cppyy cppyy_backend ${PYTHON_LIBRARIES})
 
 set_property(GLOBAL APPEND PROPERTY ROOT_EXPORTED_TARGETS cppyy)
 install(TARGETS cppyy EXPORT ${CMAKE_PROJECT_NAME}Exports DESTINATION ${runtimedir})
+
+file(COPY ${headers} DESTINATION ${CMAKE_BINARY_DIR}/include)
+install(FILES ${headers} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})