add export to xio_reject

issue: 400572

In commit 904f9d18429f6e09b20ce5b447458bd6a90fd3f8
Named "preparing to deprecate xio_get_connection from API."

A call to xio_reject was added to examples/kernel/hello_world/xio_server.c

This API was not exported in src/kernel/xio/xio_init.c fix this.

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>