ported mapstruct-spi-accessor-naming example to Gradle

[paranoidandroid522]

I had trouble getting custom accessor naming strategies to work with Gradle for lack of documentation, so I decided to port the Maven example to Gradle. See issue #1325.

[sjaakd]

@filiphr : do you have some ways to verify this one?

[sjaakd]

@paranoidandroid522 thanks for the PR!