VSOCK: fix header include in vsock_diag_test

The vsock_diag_test program directly included ../../../include/uapi/
headers from the source tree.  Tests are supposed to use the
usr/include/linux/ headers that have been prepared with make
headers_install instead.

Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>