common/LogEntry: fix warning on i386

common/LogEntry.cc: In member function 'void LogEntry::log_to_syslog(std::string, std::string)':
warning: common/LogEntry.cc:161:20: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'uint64_t {aka long long unsigned int}' [-Wformat=]
msg.c_str());
^

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