Message: Serialize fallback markers in both public/private SCE mode

This is needed to allow fallback markers for private content, e.g.
for message replies.

src/base/QXmppMessage.cpp

@@ -1953,11 +1953,6 @@ void QXmppMessage::serializeExtensions(QXmlStreamWriter *writer, QXmpp::SceMode
  d->omemoElement->toXml(writer);
  }
 #endif
-
- // XEP-0428: Fallback Indication
- for (const auto &fallback : d->fallbackMarkers) {
- fallback.toXml(writer);
- }
  }
 
  if (sceMode & QXmpp::SceSensitive) {
@@ -2151,6 +2146,15 @@ void QXmppMessage::serializeExtensions(QXmlStreamWriter *writer, QXmpp::SceMode
  d->callInviteElement->toXml(writer);
  }
  }
+
+ // serialize in private and in public part
+
+ // XEP-0428: Fallback Indication
+ // fallback markers may be used in the private part (e.g. message replies) but also in the
+ // public part (e.g. the fallback body for e2ee messages)
+ for (const auto &fallback : d->fallbackMarkers) {
+ fallback.toXml(writer);
+ }
 }
 
 struct QXmppFallbackPrivate : QSharedData {