Avoid implicit loss of precision in odd.c

What's being written to `odd_cnt` is the result of pointer arithmetics, which don't fit `int` values on 64bit machines.

This silences the following warning:

```
../../../../ext/oj/odd.c:104:26: warning: implicit conversion loses integer precision: 'long' to 'int' [-Wshorten-64-to-32]
    odd_cnt = odd - odds + 1l;
```

on platform:

```
Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
```