security/libargon2 is not jobs safe

This concurrent flaw is pretty awful but easily fixed.
The "all" target depends on "clean $(RUN) libs".
In a multijob invocation, these targets all start building at the
same time beccause there's no requirement to clean before building.
As a result, it's possible that some object files get built before the
clean target runs, which deletes the built files.

Probably this can be solved by simply removing the "clean" target which
should be unnecessary in a ports build area. I'll leave that to the
maintainer to either patch or notify upstream of this obvious makefile
flaw.