struct XmlTableRoutine: use C99 designated initializers

As in c27f8621ee et al.

Not as critical as other cases we've handled, but I figure if we're
going to add JsonbTableRoutine using TableFuncRoutine, this makes it
easier to jump around the code.

src/backend/utils/adt/xml.c

@@ -213,14 +213,14 @@ static void XmlTableDestroyOpaque(struct TableFuncScanState *state);
 
 const TableFuncRoutine XmlTableRoutine =
 {
-	XmlTableInitOpaque,
-	XmlTableSetDocument,
-	XmlTableSetNamespace,
-	XmlTableSetRowFilter,
-	XmlTableSetColumnFilter,
-	XmlTableFetchRow,
-	XmlTableGetValue,
-	XmlTableDestroyOpaque
+	.InitOpaque = XmlTableInitOpaque,
+	.SetDocument = XmlTableSetDocument,
+	.SetNamespace = XmlTableSetNamespace,
+	.SetRowFilter = XmlTableSetRowFilter,
+	.SetColumnFilter = XmlTableSetColumnFilter,
+	.FetchRow = XmlTableFetchRow,
+	.GetValue = XmlTableGetValue,
+	.DestroyOpaque = XmlTableDestroyOpaque
 };
 
 #define NO_XML_SUPPORT() \