BUILD: sample: use strtoll() instead of atoll()

BUILD: sample: use strtoll() instead of atoll()

atoll() is not portable, but strtoll() is more common. We must pass NULL
to the end pointer however since the parser must consume digits and stop
at the first non-digit char. No backport is needed as this was introduced
in 2.4-dev17 with commit 51c8ad45c ("MINOR: sample: converter: Add json_query
converter").