JAMES-3502, add functional interface annotations
diff --git a/backends-common/rabbitmq/src/main/java/org/apache/james/backends/rabbitmq/RabbitMQConfiguration.java b/backends-common/rabbitmq/src/main/java/org/apache/james/backends/rabbitmq/RabbitMQConfiguration.java
index a5155cb..0032770 100644
--- a/backends-common/rabbitmq/src/main/java/org/apache/james/backends/rabbitmq/RabbitMQConfiguration.java
+++ b/backends-common/rabbitmq/src/main/java/org/apache/james/backends/rabbitmq/RabbitMQConfiguration.java
@@ -171,6 +171,7 @@
 
         static class Builder {
 
+            @FunctionalInterface
             interface RequireSSLStrategyTrustStore {
                 RequireHostNameVerifier sslStrategy(SSLValidationStrategy strategy, Optional<SSLTrustStore> trustStore);
 
@@ -187,6 +188,7 @@
                 }
             }
 
+            @FunctionalInterface
             interface RequireHostNameVerifier {
                 ReadyToBuild hostNameVerifier(HostNameVerifier hostNameVerifier);