[alerts-dispatcher] update to build again :) also fixes a concurrent map access

go/src/infra/monitoring/messages/gatekeeper.go

Index: go/src/infra/monitoring/messages/gatekeeper.go
diff --git a/go/src/infra/monitoring/messages/gatekeeper.go b/go/src/infra/monitoring/messages/gatekeeper.go
index 8ba9eca0fdd1a6a9e66fc65b9c642ee6c848b383..e0d7a3e7addd3325e99066130c3b9d91d14e7726 100644
--- a/go/src/infra/monitoring/messages/gatekeeper.go
+++ b/go/src/infra/monitoring/messages/gatekeeper.go
@@ -28,6 +28,11 @@ type BuilderConfig struct {
 	ClosingOptional   []string `json:"closing_optional"`
 }
 
+type GatekeeperConfig struct {
+	Categories map[string]BuilderConfig  `json:"categories"`
+	Masters    map[string][]MasterConfig `json:"masters"`
+}
+
 // TreeMasterConfig is a named group of masters. e.g. chromium, or blink.
 type TreeMasterConfig struct {
 	BuildDB string   `json:"build-db"`