gprint: fix comma after ip addresses

gprint: fix comma after ip addresses

Do the same as the oprint plugin: let inet_ntop() write to a temporary
buffer, and then write that buffer content and the trailing comma to the
actual output buffer in one go.

Fixes: f04bf6794d11 ("gprint, oprint: use inet_ntop to format ip addresses")
Signed-off-by: Corubba Smith <corubba@gmx.de>
Signed-off-by: Florian Westphal <fw@strlen.de>