vim: move vim_acl_T to types.h

Also include "types.h" in os_unix.h because it declares functions that
return vim_acl_T.

src/nvim/os_unix.h

@@ -1,6 +1,7 @@
 #ifndef NVIM_OS_UNIX_H
 #define NVIM_OS_UNIX_H
 
+#include "nvim/types.h"  // for vim_acl_T
 #include "nvim/os/shell.h"
 
 /* Values returned by mch_nodetype() */