patch 8.2.0858: not easy to require Lua modules

Problem:    Not easy to require Lua modules.
Solution:   Improve use of Lua path. (Prabir Shrestha, closes #6098)

src/proto/if_lua.pro

@@ -8,4 +8,5 @@ void lua_buffer_free(buf_T *o);
 void lua_window_free(win_T *o);
 void do_luaeval(char_u *str, typval_T *arg, typval_T *rettv);
 int set_ref_in_lua(int copyID);
+void update_package_paths_in_lua(void);
 /* vim: set ft=c : */