fix incompatibilities with lua 5.2 ?

Hi,

While packaging this library for nixos in order to run neovim tests, I stumbled upon this error which I believe is due to lua 5.2(.3) incompatibility. Would it be possible to fix it please ?
```
unpacking sources
unpacking source archive /nix/store/6jwp0g5bsswgv55azirwla8w3mmj8wyn-lua-iconv-7-3.src.rock
'/nix/store/6jwp0g5bsswgv55azirwla8w3mmj8wyn-lua-iconv-7-3.src.rock' -> 'lua-iconv-7-3.src.rock'
Warning: The directory '/homeless-shelter/.cache/luarocks' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing /nix/store/ksx0538jndaskhgwgh8vbj91fa4r4xr3-luarocks-2.4.3/bin/.luarocks-wrapped with sudo, you may want sudo's -H flag.
lua_cmsgpack.c: In function 'mp_encode_lua_table_as_array':
lua_cmsgpack.c:331:18: warning: implicit declaration of function 'lua_objlen' [-Wimplicit-function-declaration]
size_t len = lua_objlen(L,-1), j;
^~~~~~~~~~
lua_cmsgpack.c: At top level:
lua_cmsgpack.c:690:30: error: array type has incomplete element type 'struct luaL_reg'
static const struct luaL_reg thislib[] = {
^~~~~~~
lua_cmsgpack.c: In function 'luaopen_cmsgpack':
lua_cmsgpack.c:697:5: warning: implicit declaration of function 'luaL_register' [-Wimplicit-function-declaration]
luaL_register(L, "cmsgpack", thislib);
^~~~~~~~~~~~~

Error: Build error: Failed compiling object lua_cmsgpack.o
gcc -O2 -fPIC -I/nix/store/bb68br3ravhcacg3hsnsjwqib3rj6b5s-lua-5.2.3/include -c lua_cmsgpack.c -o lua_cmsgpack.o
builder for ‘/nix/store/6wskmyg22i17jv7b38mznxk43my6cgpg-lua5.2-lua-cmsgpack-0.3-2.drv’ failed with ex
```