Add import for Plugin in generateGazBin.groovy

Turns out this script never worked as there was a missing import for the `Plugin` class.

Author: greenwoodma

groovy/generateGazBin.groovy

@@ -1,6 +1,7 @@
 @GrabResolver(name='gate-snapshots', root='http://repo.gate.ac.uk/content/groups/public/')
 @Grab('uk.ac.gate:gate-core:8.6-SNAPSHOT')
 import gate.*;
+import gate.creole.Plugin;
 
 // initialize GATE
 Gate.init();