Error message didn't match the actual check.  Fix that.  Compression of leaf
SP-GiST values was introduced in 11.  So, backpatch.

Discussion: https://postgr.es/m/20180810.100742.15469435.horiguchi.kyotaro%40lab.ntt.co.jp
Author: Kyotaro Horiguchi
Backpatch-through: 11