Make separate header for nr_pdcp instead of using 4G pdcp

Creates function that have nr_pdcp_ prepended to their name, and change
the signature of some functions for the 5G PDCP:

- nr_pdcp_layer_init()
- nr_pdcp_data_req()
- nr_pdcp_remove_UE()
- nr_pdcp_config_set_security()
- nr_pdcp_remove_UE()
- Make separate functions for nr_pdcp_data_req_srb/drb()

executables/lte-softmodem.c

@@ -181,6 +181,21 @@ bool sdap_data_req(protocol_ctxt_t *ctxt_p,
   abort();
 }
 
+/* hack: gtp_itf.cpp requires this empty function to be defined here */
+bool nr_pdcp_data_req_drb(protocol_ctxt_t *ctxt_pP,
+                          const srb_flag_t srb_flagP,
+                          const rb_id_t rb_id,
+                          const mui_t muiP,
+                          const confirm_t confirmP,
+                          const sdu_size_t sdu_buffer_size,
+                          unsigned char *const sdu_buffer,
+                          const pdcp_transmission_mode_t mode,
+                          const uint32_t *const sourceL2Id,
+                          const uint32_t *const destinationL2Id)
+{
+  abort();
+}
+
 /* forward declarations */
 void set_default_frame_parms(LTE_DL_FRAME_PARMS *frame_parms[MAX_NUM_CCs]);