Changes

gsm_04_08: add GSM48_BCAP_RRQ_SPARE_NETWORK_TO_MS

From network to MS, we must send spare bits that are encoded the same as
GSM48_BCAP_RRQ_FR_ONLY. Add a define, so it is obvious that those are
spare bits.

Related: OS#6657
Related: osmo-msc I7046e9244fd9d4301ee2c4df1147a619f753739c
Change-Id: I97101c977104eae82e4850d40f9abd15aa03e33e