[libbeat] fix a small linting issue (elastic#39726)

The linter complains of an `errcheck`.

libbeat/monitoring/report/report.go

@@ -128,7 +128,7 @@ func collectSubObject(cfg *conf.C) *conf.C {
 	for _, field := range cfg.GetFields() {
 		if obj, err := cfg.Child(field, -1); err == nil {
 			// on error field is no object, but primitive value -> ignore
-			out.SetChild(field, -1, obj)
+			out.SetChild(field, -1, obj) //nolint:errcheck // this error is safe to ignore
 			continue
 		}
 	}