Added header guards to files in directory extensions/*

Change-Id: If6a1642d726732fa05735c721f31b6fc7c405608
Reviewed-on: https://gerrit.libreoffice.org/9537Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>