gpst: Sort poscount output to make test #288 pass with newer Perls

Perl versions containing commit 7dc8663964c66a698d31bbdc8e8abed69bddeec3
(v5.17.5-518-g7dc8663, "Hash Function Change - Murmur hash and true per
process hash seed", 2012-11-17 14:12:04 +0100) changes the traversal
order of hashes, making it random. Sort the output to make it
predictable.

a019a750-3b5f-11e4-a286-c80aa9e67bbd