Update to Linux Kernel 4.12.4

This updates the linux-postmarketos kernel to 4.12.4, and fixes an issue where the APKBUILD was pulling down the 4.12 source but not the 4.12.x patches. I think that pulling down the major.minor source and a sep. package for patches to update to the major.minor.minor(er?) release adds unnecessary complexity since kernel.org already hosts the sources with these patches applied (e.g. in this case the 4.12.4 source)