qjson: Handle "\f"

It's valid JSON and should be handled.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/qjson.c b/qjson.c
index 483c667..e4ee433 100644
--- a/qjson.c
+++ b/qjson.c
@@ -158,6 +158,9 @@
                 case '\b':
                     qstring_append(str, "\\b");
                     break;
+                case '\f':
+                    qstring_append(str, "\\f");
+                    break;
                 case '\n':
                     qstring_append(str, "\\n");
                     break;