Make x-pack-core generate a pom file

This was a forgotten part of elastic#32180

Author: rjernst

x-pack/plugin/core/build.gradle

@@ -6,6 +6,8 @@ import java.nio.file.Paths
 import java.nio.file.StandardCopyOption
 
 apply plugin: 'elasticsearch.esplugin'
+apply plugin: 'nebula.maven-base-publish'
+apply plugin: 'nebula.maven-scm'
 
 archivesBaseName = 'x-pack-core'