generate_javadoc_for_release: set notimestamp=true

This makes incremental publishing of javadoc much simpler, as unchanged files will
no longer have spurious changes due to the hidden timestamp comment or the
'<meta name="date">' header.
diff --git a/generate_javadoc_for_release.sh b/generate_javadoc_for_release.sh
index 721983c..445926e 100755
--- a/generate_javadoc_for_release.sh
+++ b/generate_javadoc_for_release.sh
@@ -112,7 +112,7 @@
 # This might fail due to duplications in the classpath (see https://issues.apache.org/jira/browse/SLING-6766?focusedCommentId=16358298&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16358298)
 # The classpath order is unfortunately not predictable with m-j-p 3.0.0 (https://issues.apache.org/jira/browse/MJAVADOC-513)
 mvn -DexcludePackageNames="*.impl:*.internal:*.jsp:sun.misc:*.juli:*.testservices:*.integrationtest:*.maven:javax.*:org.osgi.*" \
-         org.apache.maven.plugins:maven-javadoc-plugin:3.0.0:aggregate
+         org.apache.maven.plugins:maven-javadoc-plugin:3.0.0:aggregate -Dnotimestamp=true
 popd
 
 echo "Generated Javadocs can be found in $WORKDIR/target/site/apidocs/"