Stop using <stropts.h>

In Vim, at least the constant `I_PUSH` is used from <stropts.h>, but
Neovim doesn't seem to use anything from said header.

Besides that, POSIX.1-2008[1] marks this header as obsolescent, and
there don't seem to be many platforms that even have it.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stropts.h.html#tag_13_52_11

src/nvim/os_unix.c

@@ -44,10 +44,6 @@
 #include "nvim/os/signal.h"
 #include "nvim/msgpack_rpc/helpers.h"
 
-#ifdef HAVE_STROPTS_H
-# include <stropts.h>
-#endif
-
 #ifdef HAVE_SELINUX
 # include <selinux/selinux.h>
 static int selinux_enabled = -1;