MinGW compilation warnings in libiberty's xstrndup.c

[Eli Zaretskii]

> From: DJ Delorie <dj@redhat.com>
> Cc: gcc-patches@gcc.gnu.org, gdb-patches@sourceware.org
> Date: Fri, 26 May 2017 17:34:12 -0400
> 
> 
> Please try this patch:

Seems to work fine, thanks.