Build fails on OS X/clang

[Al Cutter]

clang -Wall -Werror -I /Users/travis/build/google/certificate-transparency/../install/include -o objecthash_test objecthash_test.c objecthash.c -L /Users/travis/build/google/certificate-transparency/../install/lib -ljson-c -lcrypto
objecthash.c:75:23: error: format specifies type 'long' but the argument has type 'int64_t' (aka 'long long') [-Werror,-Wformat]
  sprintf(buf, "%ld", i);
                ~~~   ^
                %lld

[Ben Laurie]

Fixed by #12.