add ElementScrollBehavior enum to BUCK so it's included in the release artifact like it was in 2.X

Fixes #2954

Author: lukeis

java/client/src/org/openqa/selenium/BUCK

@@ -40,6 +40,7 @@ java_library(name = 'core',
     'UnexpectedAlertBehaviour.java',
     'WebDriver.java',
     'WebElement.java',
+    'internal/ElementScrollBehavior.java',
     'internal/HasIdentity.java',
     'internal/Killable.java',
     'internal/Locatable.java',