lib key to Platypus takes a single string

On my machine (macOS), dynamic_lib returns more than one, although they are the same names:

```
$ perl -MAlien::Sodium -le 'print join qq(\n), Alien::Sodium->dynamic_libs'
/usr/local/perls/perl-5.38.2/lib/site_perl/5.38.2/darwin-2level/auto/share/dist/Alien-Sodium/dynamic/libsodium.dylib
/usr/local/perls/perl-5.38.2/lib/site_perl/5.38.2/darwin-2level/auto/share/dist/Alien-Sodium/dynamic/libsodium.26.dylib
```

We only need one of them to do away with:

    Odd number of elements in hash assignment at /usr/local/perls/perl-5.38.2/lib/site_perl/5.38.2/darwin-2level/FFI/Platypus.pm line 35.

lib/Sodium/FFI.pm

@@ -40,7 +40,7 @@ push @EXPORT_OK, qw(
 
 our $ffi;
 BEGIN {
-    $ffi = FFI::Platypus->new(api => 1, lib => Alien::Sodium->dynamic_libs);
+    $ffi = FFI::Platypus->new(api => 1, lib => (Alien::Sodium->dynamic_libs)[0]);
     $ffi->bundle();
 }
 # All of these functions don't need to be gated by version.