Standalone GUI doesn't need to pull in all the generic disks

A regression from my generic disk code makes the standalone GUI fail to compile. It pulls in sys_unix.cpp just for SysAddSerialPrefs(), which then wants disk_sparsebundle_factory() and friends.

It's rather silly to include disk_sparsebundle.cpp in just to satisfy that. So my preferred fix is to just #ifdef out the factory functions if we're building standalone. Sound ok?