openair2/COMMON/rrc_messages_types.h has mostly 4G types. The only 5G
(gNB) RRC type is gNB_RrcConfigurationReq, which we can move to a header
in the corresponding directory (where it is used).
This allows to separate 5G gNB RRC module from the 4G types in
rrc_messages_types.h in the next commit.