[jsonapi] Prevent browsers to cache playlist tracks

The tracks of a smart playlist might change between library rescans.
Allowing them to be cached based on the last rescan timestamp
("Last-Modified" header in the response) leads to potentially showing
incorrect track listing if a cached version is used. Thus the response
for playlist tracks should never be cached by the browser (this is
achieved with setting "Cache-Control" header to "no-store").

src/httpd.h

@@ -107,6 +107,9 @@ httpd_request_not_modified_since(struct evhttp_request *req, time_t mtime);
 bool
 httpd_request_etag_matches(struct evhttp_request *req, const char *etag);
 
+void
+httpd_response_not_cachable(struct evhttp_request *req);
+
 /*
  * Gzips an evbuffer
  *