put the service monitor for the blackbox exporter in the configured namespace

jsonnet/kube-prometheus/blackbox-exporter/blackbox-exporter.libsonnet

@@ -194,6 +194,7 @@
           kind: 'ServiceMonitor',
           metadata: {
             name: 'blackbox-exporter',
+            namespace: $._config.namespace,
             labels: bb.assignLabels
           },
           spec: {