Added header guards to files in directories sc/, sd/, slideshow/, and solenv/

Change-Id: Ib4577761644fe72f5ddb3c5060e629bf94d6e6e2
Reviewed-on: https://gerrit.libreoffice.org/9583Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>