cgen: fix struct init with update expr (fix #15595) (#15603)

Author: yuyi98

vlib/v/gen/c/struct.v

@@ -195,7 +195,9 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 				if is_update_tmp_var {
 					g.write(tmp_update_var)
 				} else {
+					g.write('(')
 					g.expr(node.update_expr)
+					g.write(')')
 				}
 				if node.update_expr_type.is_ptr() {
 					g.write('->')

vlib/v/tests/struct_init_with_update_test.v

@@ -30,3 +30,21 @@ fn test_struct_init_with_update_expr() {
 	assert o.height == 175
 	assert o.age == 21
 }
+
+struct Foo {
+	s string
+	n int
+}
+
+fn test_struct_init_with_update_expr2() {
+	f := &Foo{
+		s: 'AA'
+	}
+	b := Foo{
+		...*f
+		n: 3
+	}
+	println(b)
+	assert b.s == 'AA'
+	assert b.n == 3
+}