luna-prefs.pc: Add requires for json-c

* otherwise some components can fail to find json.h included from
  lunaprefs.h:26, because they don't have /usr/include/json in CFLAGS
* it wasn't needed with cjson, because includes were written as
  <cjson/json.h> and /usr/include was always searched

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>