src: json: remove spaces

Remove all the spaces from the JSON output to reduce the size
of the output string, this also provides a consistent output
in table, chain, rule and set.

As Stephen Hemminger suggested, better to squash the output to
consume as less bytes as possible.

Signed-off-by: Alvaro Neira Ayuso <alvaroneay@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>