fix(build): distinguish vim.mpack from global require'mpack'

problem: the api of vim.mpack is not compatible with a system provided mpack
solution: don't require 'mpack' directly from the system path

src/mpack/lmpack.c

@@ -1171,9 +1171,6 @@ static const luaL_reg mpack_functions[] = {
   {NULL, NULL}
 };
 
-#ifdef NLUA_WIN32
-  __declspec(dllexport)
-#endif
 int luaopen_mpack(lua_State *L)
 {
   /* Unpacker */

src/mpack/lmpack.h

@@ -1,6 +1,3 @@
 #include <lua.h>
 
-#ifdef NLUA_WIN32
-  __declspec(dllexport)
-#endif
 int luaopen_mpack(lua_State *L);