[RFC] Add missing stdarg.h include.  Fixes build issue

This is a trivial change.  On my Linux machine with gcc 4.1.2, this include is required.  WIthout it, I get the following build error (while using d5a7947e8b3fc9b8f45ad1a0f0380e22e67fe6d1):

```
In file included from /rd/gen/lxd/do_not_delete/devel/neovim/src/nvim/message.h:27,
                 from /rd/gen/lxd/do_not_delete/devel/neovim/src/nvim/buffer.c:57:
/rd/gen/lxd/do_not_delete/devel/neovim/build/include/message.h.generated.h:79: error: expected declaration specifiers or '...' before 'va_list'
```

I have another machine with gcc 4.7, which doesn't require this.  So I'm guessing this might be related to the oldness of 4.1?