ci: Add generic webhook trigger

This will allow `updatecheck` to trigger builds by sending an HTTP POST
request when it finds an update for the _kernel_ package.

ci/Jenkinsfile

@@ -1,6 +1,22 @@
 pipeline {
     agent none
 
+    triggers {
+        GenericTrigger(
+            causeString: 'Webhook Trigger',
+            genericVariables: [[
+                key: 'nvr',
+                value: '$[*].builds[*].nvr',
+            ]],
+            printContributedVariables: true,
+            printPostContent: true,
+            regexpFilterExpression: 'kernel-.*',
+            regexpFilterText: '$nvr',
+            silentResponse: true,
+            tokenCredential: 'webhook-trigger',
+        )
+    }
+
     stages {
         stage('RPM') {
             matrix {