add SpiNNaker_PDP2 to automatic make

[Christian-B]

Please merge this in after

• [x] https://github.com/SpiNNakerManchester/SpiNNaker_PDP2/tree/update_to_ST6 and any other it depends on like
• [x] https://github.com/SpiNNakerManchester/spinn_common/tree/update_to_ST6

have been merged in

[Christian-B]

Automatic make silently ignores any directories is does not find so adding this will not require anyone to have PDP2