rx: Hide the rx_packet.h

rx: Hide the rx_packet.h

Hide the rx_packet.h, and hence the rx_packet structure from
application view. rx_packet.h is currently still installed, and is
included directly by RX security classes, to reduce the per-packet
overhead there.

Change-Id: I269ccf4405a8f83cab4b0392b830bc1f36471c29
Reviewed-on: http://gerrit.openafs.org/6182
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>