partutil: Explicitly use PART_TYPE_UNKNOWN

partutil: Explicitly use PART_TYPE_UNKNOWN

Previously we were comparing PartitionScheme values against the undefined enum
value -1, which causes a warning with gcc 4.5. Define -1 explicitly as
"PART_TYPE_UNKNOWN" and only use that in the code, which is cleaner.

Thanks to Marti Raudsepp for pointing this out!