rockspec: LIBUV: specify library='uv'
rockspec: LIBUV: specify library='uv'

This is required for LuaRocks to find libuv on Windows.

Ref: https://github.com/neovim/neovim/pull/10292#issuecomment-506610299

luvit/luv,luvit/luv,zhaozg/luv,zhaozg/luv