lib: add printbuf.h API from json

This API proposes to use the sprintbuf() method to facilitate
the forge of buffers in json.

Note that this buffer is limited to 128 bytes, so it can not
be used everywhere as it without any checks.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

lib/json.h

@@ -13,6 +13,7 @@ extern "C" {
 #include "command.h"
 #include "printfrr.h"
 #include <json-c/json.h>
+#include <json-c/printbuf.h>
 
 /*
  * FRR style JSON iteration.