lua-packages/luasocket: Fix wrong platforms value

This has surfaced due to 505d7bea and
the fixup commit 77f5a50c

.

The individial platform attributes are a list rather than single
elements, so in this case we got:

with platforms; [darwin linux freebsd illumos]

Which results in:

[ ["x86_64-darwin"]
  ["i686-linux" "x86_64-linux" "armv5tel-linux" "armv6l-linux"
   "armv7l-linux" "aarch64-linux" "mips64el-linux"]
  ["i686-freebsd" "x86_64-freebsd"]
  ["x86_64-solaris"]
]

So if you don't have allowBroken set in nixpkgs config, you end up with
an evaluation error because meta.platforms is expected to be a list
instead of a list of lists which we got here.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @mornfall, @vcunat