POM: Define GPG key to use when deploying.

Change-Id: I0080071d1e404a8db7f068559ddb82e1dda63d60
diff --git a/pom.xml b/pom.xml
index 273f382..21baed2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -201,6 +201,9 @@
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-gpg-plugin</artifactId>
             <version>${maven-gpg-plugin.version}</version>
+            <configuration>
+              <keyname>code@mulk.eu</keyname>
+            </configuration>
             <executions>
               <execution>
                 <id>sign-artifacts</id>