add 'make etags' to generate etags

Change-Id: Id87f71b6d31720173f0c5ec2a4fc395065223b79
Reviewed-on: https://gerrit.libreoffice.org/12300
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>