Adaptions for API change in json-c v0.13

Lift restriction on json-c <= 0.12.1

ipc-server: Acquire ownership of referenced json_object properly

When adding a referenced json_object with an unknown lifetime to
another json_object, it must be done with a wrapped call to
json_object_get() to acquire the ownership of that json_object.

include/swaygrab/json.h

@@ -1,4 +1,4 @@
-#include <json-c/json.h>
+#include "sway_json_helper.h"
 #include "wlc/wlc.h"
 
 void init_json_tree(int socketfd);