MinGW has no strtok_r, so we need a declaration in sysemu/os-win32.h.
We must also fix the include statements in util/envlist.c to include
that file.

We currently don't need an implementation of strtok_r because the
code is compiled but not linked for MinGW.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>