json: Fix compile error

json: Fix compile error

Commit 9e45a28ca467f ("src: honor /etc/services") broke compiling with
JSON support enabled: inet_service_type_print() is not suited for
converting inet_service datatype into JSON at all.

In order to avoid having to replicate the port value resolving into
human-readable name in inet_service_type_json(), just return a numeric
value. At least for JSON output, this probably makes most sense either
way since the output is expected to be parsed by scripts which have an
easier time with numers than names anyway.

Fixes: 9e45a28ca467f ("src: honor /etc/services")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>