Fix the PC version

[mariari]

I had similar issues to a report that was described here:

https://elixirforum.com/t/compile-error-in-syslog-since-upgrading-to-erlang-27/66606

% rebar3 pc compile
===> Verifying dependencies...
make: 對「libsodium」無需做任何事。
===> Uncaught error in rebar_core. Run with DIAGNOSTIC=1 to see stacktrace or consult rebar3.crashdump
===> When submitting a bug report, please include the output of `rebar3 report "your command"`

For those with the issue, clearing out the _build directory may be needed.

Note I did not test this patch on the v1.2.1 release, instead i tested it on a version of enacl with a vendored libsodium. If you are interested in the vendored version let me know as I can prepare a patch for upstreaming.