osd: MissingLoc::get_all_missing is const

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2368 from dachary/wip-pg-const

osd: MissingLoc::get_all_missing is const

Reviewed-by: Sage Weil <sage@redhat.com>