explorer: Build on Darwin versions prior to 8.0

[Phil Krylov]

Hello,

In Darwin versions prior to 8.0, the DiskArbitration framework was a
private framework, that is, the headers were not included in SDK and
the API was changing in every version. This patch fixes the build in
these OS versions.

-- Ph.

[Phil Krylov]

Sorry guys, wrong list ;-)

-- Ph.