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

CMakeLists.txt

@@ -2080,7 +2080,7 @@ add_executable(nr_srssim
   )
 
 target_link_libraries(nr_srssim PRIVATE
-  -Wl,--start-group UTIL SIMU PHY_COMMON PHY_NR_COMMON PHY_NR PHY_NR_UE SCHED_NR_LIB SCHED_NR_UE_LIB MAC_UE_NR MAC_NR_COMMON nr_rrc L2_NR -lz -Wl,--end-group
+  -Wl,--start-group UTIL SIMU PHY_COMMON PHY_NR_COMMON PHY_NR PHY_NR_UE SCHED_NR_LIB SCHED_NR_UE_LIB MAC_UE_NR MAC_NR_COMMON -lz -Wl,--end-group
   m pthread ITTI dl nr_ue_phy_meas physim_common
   )