um: writev needs <sys/uio.h>

vector_user.c doesn't compile without this for me.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_user.c

@@ -25,6 +25,7 @@
 #include <linux/if_packet.h>
 #include <sys/socket.h>
 #include <sys/wait.h>
+#include <sys/uio.h>
 #include <linux/virtio_net.h>
 #include <netdb.h>
 #include <stdlib.h>