[Pianod] Not building with json-c 0.12

[Ian Scott]

Pianod fails to build with json-c 0.12. Downgrading to 0.11 made it
compile successfully.

>From the json-c ChangeLog:
>  * Remove the old libjson.so name compatibility support.  The library is
>      only created as libjson-c.so now and headers are only installed 
>      into the ${prefix}/json-c directory.