LU-5504 utils: add const qualifier to changelog accessors.

LU-5504 utils: add const qualifier to changelog accessors.

The following accessors don't need to modify their argument:
changelog_rec_size(), changelog_rec_name(), changelog_rec_snamelen(),
changelog_rec_sname().
Make their prototype more rigorous by adding a "const" qualifier.

Signed-off-by: Thomas Leibovici <thomas.leibovici@cea.fr>
Change-Id: I7915a2487b2b772482c777c4c19ec947b7015b74
Reviewed-on: http://review.whamcloud.com/11517
Tested-by: Jenkins
Reviewed-by: frank zago <fzago@cray.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>