cgen: fix generic struct to string (#10191)

vlib/v/tests/generics_struct_to_string_test.v

@@ -0,0 +1,33 @@
+struct Info<T> {
+	data T
+}
+
+fn get_info<T>(res Info<T>) string {
+	return '$res'
+}
+
+fn test_generic_struct_to_string() {
+	mut ret := get_info(Info<bool>{true})
+	println(ret)
+	assert ret.contains('data: true')
+
+	ret = get_info(Info<int>{123})
+	println(ret)
+	assert ret.contains('data: 123')
+
+	ret = get_info(Info<f32>{f32(2.2)})
+	println(ret)
+	assert ret.contains('data: 2.2')
+
+	ret = get_info(Info<f64>{2.2})
+	println(ret)
+	assert ret.contains('data: 2.2')
+
+	ret = get_info(Info<string>{'aaa'})
+	println(ret)
+	assert ret.contains("data: 'aaa'")
+
+	ret = get_info(Info<u64>{u64(234)})
+	println(ret)
+	assert ret.contains('data: 234')
+}