c11/threads: Include thr/xtimec.h for xtime definition when building with MSVC.

c11/threads: Include thr/xtimec.h for xtime definition when building with MSVC.

MSVC has been including a xtime definition in thr/xtimec.h ever since
MSVC 2013 (which is the minimum we require for building Mesa), and
including it prevents duplicate definitions when it gets included by
LLVM.

In fact, it looks that MSVC has been including a partial C11 threads
implementation too for some time, which we should consider migrating to
once we eliminate the use of _MTX_INITIALIZER_NP in our tree.

Thanks to the anonymous helper from
https://bugs.freedesktop.org/show_bug.cgi?id=100201#c4 for spotting
this.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100201
CC: "17.0" <mesa-stable@lists.freedesktop.org>