devel/sem: Declare libzstd shlib dependency

[olgeni]

Summary

The binary links libzstd.so.1 (via the zstd-sys crate, which uses
pkg-config to pick up the system libzstd by default), but the port
does not declare the runtime dependency.

Detected by poudriere stage-qa:

Warning: sem-0.21.0 will be rebuilt as it misses libzstd.so.1
which no dependency provides. It is likely (silently) failing
testport/stage-qa.

Add LIB_DEPENDS=libzstd.so:archivers/zstd and bump PORTREVISION.

Test Plan

poudriere testport on 15.1-amd64 - clean build, stage-qa no longer
reports the missing libzstd.so.1 shlib, package installs and
deinstalls cleanly.

[adamw]

Great catch! Thank you, Jimmy!