printer json REFACTOR of dynamic flag

... because the plugin must set the flag.
diff --git a/src/printer_json.c b/src/printer_json.c
index 40c9d05..dea32d3 100644
--- a/src/printer_json.c
+++ b/src/printer_json.c
@@ -332,7 +332,7 @@
 static LY_ERR
 json_print_value(struct jsonpr_ctx *ctx, const struct lyd_value *val)
 {
-    ly_bool dynamic = 0;
+    ly_bool dynamic;
     const char *value = val->realtype->plugin->print(ctx->ctx, val, LY_VALUE_JSON, NULL, &dynamic, NULL);
 
     /* leafref is not supported */