sysutils/kvmclock: update to 0.0.0-a4

[jrm]

Looks fine from a ports perspective [1,2]. Dave is away until the 26th, so if you want me to commit as is now I think he would be alright with a basic update like this. I assume it's been runtime tested.

[1] AFAICT, the KMOD variable isn't doing anything.
[2] I know some other port committers would complain about pkg-descr being only one sentence (and links). It might be helpful to add a sentence or two describing what kvmclock is.

[jrm]

I kldloaded on 13amd64 with no problems.