tools/virtio: make asm/barrier.h self contained

We are using abort() so let's include stdlib.h
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/asm/barrier.h

 /* SPDX-License-Identifier: GPL-2.0 */
+#include <stdlib.h>
 #if defined(__i386__) || defined(__x86_64__)
 #define barrier() asm volatile("" ::: "memory")
 #define virt_mb() __sync_synchronize()