Add error.message mapping for system.syslog

salt/elasticsearch/templates/component/elastic-agent/logs-system.syslog@custom.json

@@ -0,0 +1,22 @@
+{
+  "template": {
+    "mappings": {
+      "properties": {
+        "error": {
+          "properties": {
+            "message": {
+              "type": "match_only_text"
+            }
+	  }
+	}
+      }
+    }
+  },
+  "_meta": {
+    "package": {
+      "name": "system"
+    },
+    "managed_by": "fleet",
+    "managed": true
+  }
+}