checker: fix json decoder with generic struct (#14700)

vlib/json/json_decode_with_generic_test.v

@@ -0,0 +1,24 @@
+import json
+
+struct Result<T> {
+	ok     bool
+	result T
+}
+
+struct User {
+	id       int
+	username string
+}
+
+fn func<T>() ?T {
+	text := '{"ok": true, "result":{"id":37467243, "username": "ciao"}}'
+	a := json.decode(Result<T>, text)?
+	return a.result
+}
+
+fn test_decode_with_generic_struct() ? {
+	ret := func<User>()?
+	println(ret)
+	assert ret.id == 37467243
+	assert ret.username == 'ciao'
+}