markused: fix missing builtin__memdup definition, when an option struct field is used (fix #25801) (#25802)

Author: spytheman

vlib/v/gen/c/testdata/memdup_used_by_program_with_struct_option_fields.c.must_have

@@ -0,0 +1 @@
+voidptr builtin__memdup(voidptr src, isize sz);

vlib/v/gen/c/testdata/memdup_used_by_program_with_struct_option_fields.vv

@@ -0,0 +1,15 @@
+struct Foo {
+	data &int
+}
+
+struct FooHolder {
+	foo ?Foo
+}
+
+struct Program {
+	foo ?Foo
+}
+
+fn main() {
+	_ := Program{}
+}

vlib/v/markused/walker.v

@@ -1428,7 +1428,7 @@ fn (mut w Walker) mark_resource_dependencies() {
 		w.fn_by_name('malloc')
 		w.fn_by_name('tos3')
 	}
-	if w.uses_memdup {
+	if w.uses_memdup || w.used_none > 0 {
 		w.fn_by_name('memdup')
 	}
 	if w.uses_debugger {