cgpt: fix simplified hybrid MBR

Commit dbf566e2 switched from creating a hybrid MBR with two protective
partitions to just one, opting to cover the end of the disk since that
included the real data. However it turns out the hybrid MBR is not
probably detected as such by Linux in this case, the protective
partition needs to cover the beginning of the disk instead.
Merge pull request #23 from marineam/fix

cgpt: fix simplified hybrid MBR