Fix compile warning in db_stress.cc on Mac

Summary:
Fix the following compile warning in db_stress.cc on Mac
tools/db_stress.cc:1688:52: error: format specifies type 'unsigned long' but the argument has type '::google::uint64' (aka 'unsigned long long') [-Werror,-Wformat]
    fprintf(stdout, "DB-write-buffer-size: %lu\n", FLAGS_db_write_buffer_size);
                                           ~~~     ^~~~~~~~~~~~~~~~~~~~~~~~~~
                                           %llu

Test Plan:
make