bitset-iter: define count_leading_zeros<unsigned long long>
bitset-iter: define count_leading_zeros<unsigned long long>

This was missing. Presumingly, the compiler found
count_leading_zeros<unsigned long> for it as `long long`
is same as `long` on the supported architectures.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar