Turn off a Gradle warning about Gradle 5.0 incompatibility

The issue here is a planned change to how "publishing" blocks work.
Per
<https://docs.gradle.org/4.9/userguide/publishing_maven.html#publishing_maven:deferred_configuration>,
the right way to prepare for this change is to enable it and check for
unexpected changes in what gets published to a local repository.  I
have done this, and find no unexpected changes.

So we are actually ready for Gradle 5.0; the warning is a false
positive for us.  Leaving the future change enabled means we won't
keep seeing this warning.  It also means that any further changes to
our use of "publishing" will be tested under that future change, which
is a good way to avoid surprises later.

settings.gradle

@@ -35,3 +35,5 @@ include(
 	'com.ibm.wala.util',
 	'com.ibm.wala_feature',
 )
+
+enableFeaturePreview('STABLE_PUBLISHING')