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.h

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