Fix nr_srssim linking problems when building with E2 support

In this case, there are a number of linker errors:

    /usr/bin/ld: libL2_NR.a(rrc_gNB.c.o): in function `nr_rrc_transfer_protected_rrc_message':
    /home/richie/oai/openair2/RRC/NR/rrc_gNB.c:330:(.text+0x6f6): undefined reference to `signal_rrc_msg'

However, nr_srssim does not need RRC, while it is being linked in. Thus,
link without nr_rrc and L2. For the remaining two linker problems:

- provide a definition of NR_IF_Module_init() as in most other
  simulators
- move rrc_get_max_nr_csrs() to nr_common.c. It is already declared in
  the corresponding header, and itself does not have anything to do with
  RRC radio configuration, therefore it does not belong to
  nr_radio_config.c

openair1/SIMULATION/NR_PHY/srssim.c

@@ -68,6 +68,8 @@ nrUE_params_t *get_nrUE_params(void)
 channel_desc_t *UE2gNB[MAX_MOBILES_PER_GNB][NUMBER_OF_gNB_MAX];
 configmodule_interface_t *uniqCfg = NULL;
 
+NR_IF_Module_t *NR_IF_Module_init(int Mod_id) { return (NULL); }
+
 void e1_bearer_context_setup(const e1ap_bearer_setup_req_t *req)
 {
   abort();