Prepare the project for json-glib.

This required writing a special CMake find module.

src/ld-document.c

@@ -9,6 +9,7 @@
  */
 
 #include <gtk/gtk.h>
+#include <json-glib/json-glib.h>
 
 #include "config.h"
 

src/ld-library.c

@@ -11,6 +11,7 @@
 #include <string.h>
 
 #include <gtk/gtk.h>
+#include <json-glib/json-glib.h>
 
 #include "config.h"