Bump "Error Prone" to 2.0.15

and fix a few things :)

smack-experimental/src/main/java/org/jivesoftware/smackx/hoxt/packet/AbstractHttpOverXmpp.java

@@ -44,6 +44,7 @@ public abstract class AbstractHttpOverXmpp extends IQ {
         this.version = Objects.requireNonNull(builder.version, "version must not be null");
     }
 
+    @Override
     protected IQChildElementXmlStringBuilder getIQChildElementBuilder(IQChildElementXmlStringBuilder xml) {
         IQChildElementXmlStringBuilder builder = getIQHoxtChildElementBuilder(xml);
         builder.optAppend(headers);