Add example for mapstruct with Gradle 5

The example for Gradle 5 does NOT seem to work with Gradle 5. From the release notes: Release notes Gradle 5 states: Gradle will no longer automatically apply annotation processors that are on the compile classpath — use CompileOptions.annotationProcessorPath instead.
Specifically the "options.compilerArgs = [
'-Amapstruct.suppressGeneratorTimestamp=true" part.

See also my post on StackOverflow: https://stackoverflow.com/questions/56170222/migrating-from-gradle-4-to-5-how-to-get-mapstruct-1-20-final-working-with-it

从 Gradle 5 示例开始，将其 annotation processor 设置与 Gradle 5 发布说明中的要求以及所引用的 Stack Overflow 报告进行比较。更新该示例，使其能够与 Gradle 5 一起工作，包括 issue 中显示的编译器参数，并验证该示例能够成功构建。