Current support for spl-0.7.13 and later (including HEAD) is up to 5.0-rc3 (See the "see also" link). Current stable kernel sources are 4.19.x, triggering a build failure with the stable kernel.
working on getting it updated, but it's very low priority, because 0.7.13 is exactly git tip of zfs-0.7-release branch and it's going away with 0.8 release anyway. just install 0.7.13. I'll leave this open for now.
We removed sys-kernel/spl some time ago.