net/virtio-user: fix build with gcc 6

net/virtio-user: fix build with gcc 6

The error is reported using test build script:

$ scripts/test-build.sh x86_64-native-linuxapp-gcc
...
drivers/net/virtio/virtio_user_ethdev.c:345:2: error:
  this ‘if’ clause does not guard... [-Werror=misleading-indentation]
  if (rte_kvargs_count(kvlist, VIRTIO_USER_ARG_PATH) == 1)
    ^~

Fixes: 404bd6bfe360 ("net/virtio-user: fix return value not checked")

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>