[-] Make json-c a required library

Already kind of was required, as it was required by Spotify, Chromecast and the web api

src/httpd_jsonapi.c

@@ -29,7 +29,7 @@
 #ifdef HAVE_CONFIG_H
 # include <config.h>
 #endif
-#include <json.h>
+
 #include <regex.h>
 #include <stdio.h>
 #include <string.h>