paging: Fix encoding of PAGING TYPE 3 Rest Octets
paging: Fix encoding of PAGING TYPE 3 Rest Octets

The RR PAGING TYPE 3 Rest Octets IE contains (among other things) the
channel type needed for Mobile Identity 3 + 4 in the paging message.

We did not only "forget" to encode those channel type needed field, but
we have a completely wrong definition of those rest octets in
libosmocore/include/gsm/protocol/gsm04_08.h "struct gsm48_paging3"

Change-Id: I3a0bca6707ce95b68459c89f5b2b07f1590a1ab3
Closes: OS#2994

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts