osd: fix printf warning on pg_log_entry_t::get_key_name

warning: osd/osd_types.cc:1716:76: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'version_t {aka long long unsigned int}' [-Wformat]
warning: osd/osd_types.cc:1716:76: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'version_t {aka long long unsigned int}' [-Wformat]

Signed-off-by: Sage Weil <sage@inktank.com>