jenkins/update2head: fixed coding convention

the curly brackets shall be used even on if statements with
only one line

Signed-off-by: Marc Mattmueller <marc.mattmueller@netmodule.com>

Jenkinsfile_update2head

@@ -28,11 +28,12 @@ pipeline {
     stage('prepare') {
       steps {
         script {
-          if(params.UPDATE_NM_PARTS)
+          if(params.UPDATE_NM_PARTS) {
             currentBuild.displayName += "-nm"
-          if(params.UPDATE_COMMUNITY_PARTS)
+          }
+          if(params.UPDATE_COMMUNITY_PARTS) {
             currentBuild.displayName += "-community"
-
+          }
           printJobParameters()
           prepareUpdate()
         }