[ntuple] RNTupleSerialize: do not use `abs()` in feature flags deserialization
[ntuple] RNTupleSerialize: do not use `abs()` in feature flags deserialization

It seems that system headers for Clang 10.0.1 in EL 8.3 x86_64 do not provide
an overload for `abs(std::uint64_t)`, triggering the diagnostic below. Using
`(f < 0) ? -f : f` instead.

```
RNTupleSerialize.cxx:734:26: warning: absolute value function 'abs' given an argument of type 'std::int64_t' (aka 'long') but has parameter of type 'int' which may cause truncation of value [-Wabsolute-value]
      flags.emplace_back(abs(f));
```

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root