libsystemd-journal: export new function, increase library version

Since sd_journal_reliable_fd wasn't exported before, it is as if
it was added now. Library "current" number must be bumped.

michich> Someone links with the fixed version and produces a RPM with
         his program. The RPM will happily install on a system with an
         old systemd version (the deps will appear fine), but the
         program will fail to run.