convert include/svx/xmlexchg.hxx from String to OUString
convert include/svx/xmlexchg.hxx from String to OUString

Change-Id: I45a71c4b5983dcd2f19d0bc10ed008a09094eb0a

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core