tools/virtio: add a missing )

Fixes the following build failure:

 cc -g -O2 -Wall -I. -I ../../usr/include/ -Wno-pointer-sign
   -fno-strict-overflow -fno-strict-aliasing -fno-common -MMD
   -U_FORTIFY_SOURCE   -c -o virtio_test.o virtio_test.c
 virtio_test.c: In function ‘run_test’:
 virtio_test.c:176:7: error: expected ‘)’ before ‘r’
         r = -1;
         ^

Fixes: 53c18c99 (virtio_test: verify if virtqueue_kick() succeeded)
Cc: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>