Build: Clean buildSrc on gradle clean

Same change as elastic/elasticsearchelastic/elasticsearch#14785 but for xplugins.

Original commit: elastic/x-pack-elasticsearch@02e8247686

build.gradle

@@ -16,6 +16,11 @@ if (hasProperty('projectsPrefix') == false) {
   }
 }
 
+task clean(type: GradleBuild) {
+  buildFile = 'buildSrc/build.gradle'
+  tasks = ['clean']
+}
+
         <repository>
             <id>elasticsearch-releases</id>
             <url>http://maven.elasticsearch.org/releases</url>