Fix compilation using mingw.org's MinGW

[Eli Zaretskii]

> Cc: lrn1986@gmail.com, gdb-patches@sourceware.org
> From: Pedro Alves <palves@redhat.com>
> Date: Tue, 30 Apr 2019 18:58:42 +0100
> 
> In fact, your patch is doing exactly that.  common-defs.h is garanteed
> to be included before any started header.  That's exactly why I suggested
> that spot.

You were talking about the patch for master, whereas I was talking
about the patch for the release branch.