Using *unpacked* 'struct osmo_gcr_parsed' in the MNCC PDUs makes
the protocol even more complicated than it currently is, and
moreover complicates implementing MNCCv8 in the ttcn3-sip-test.
Replace 'struct osmo_gcr_parsed' in 'struct gsm_mncc' with a
fixed-length buffer, which is supposed to hold the Global Call
Reference encoded as per 3GPP TS 29.205.
Check / indicate presence of GCR using the MNCC_F_GCR flag.
Change-Id: Iaff46732948f8f5d03e42f17c35cbac8a80af49b
Fixes: Id40d7e0fed9356f801b3627c118150055e7232b1
Related: OS#5164, OS#5282