qemu-barrier: Fix compilation on i386 hosts

Commit 610b823e uses QEMU_GNUC_PREREQ
on i386 hosts.

That macro is defined in qemu-common.h which is not always included
before qemu-barrier.h, so compilation on i386 hosts was broken.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>