reference properly-namespaced ConfigurationError when failing

Previous code was failing to resolve ConfigurationError at runtime,
because there is no ConfigurationError on the lexical scope or included
into the LSCL AST.

Instead of emitting the helpful error intended, it blew up with a
NameError that adds noise to the problem instead of being helpful as
intended.

Fixes elastic#9343

logstash-core/lib/logstash/compiler/lscl.rb

@@ -181,7 +181,7 @@ def validate!
       duplicate_values = find_duplicate_keys
 
       if duplicate_values.size > 0
-        raise ConfigurationError.new(
+        raise ::LogStash::ConfigurationError.new(
           I18n.t("logstash.runner.configuration.invalid_plugin_settings_duplicate_keys",
             :keys => duplicate_values.join(', '),
             :line => input.line_of(interval.first),