recognize koush superuser when built from source

unless specified otherwise, koush superuser when built from source uses "com.thirdparty.superuser" so it doesn't conflict with the Play store version. for example, the latest stable android-x86 build (4.4-r1) uses this package name for the built-in version, and LilDebi doesn't see it, prompting the user to install superuser.

https://github.com/koush/Superuser/blob/master/README.md#configuring-the-package-name