RC: forward declarations and link physims better

The simulators have been changed: due to linker errors that would arise
otherwise, a lot of functions are defined in them to avoid such errors.
Some functions are also defined in header files; in this commit, we
define most functoins in nr_dummy_functions.c, which is compiled in, not
by including the C file, but by adding it to the target in
CMakeLists.txt

- nr_dlsim+nr_ulsim need NR_IF_Module_init(), so it cannot be in
  nr_dummy_functions, but the others need it
- Link correcr NR_IF_Module_init() into these simulators, and reduce
  overall link list
- Correctly link remaining simulators

openair2/F1AP/f1ap_cu_rrc_message_transfer.c

@@ -37,6 +37,7 @@
 #include "f1ap_cu_rrc_message_transfer.h"
 #include "common/ran_context.h"
 #include "openair3/UTILS/conversions.h"
+#include "pdcp.h"
 
 /*
     Initial UL RRC Message Transfer

openair2/F1AP/f1ap_cu_ue_context_management.c

@@ -43,6 +43,7 @@
 #include "rrc_eNB_GTPV1U.h"
 #include "openair2/RRC/NR/rrc_gNB_NGAP.h"
 #include <openair3/ocp-gtpu/gtp_itf.h>
+#include "pdcp.h"
 
 static void setQos(F1AP_NonDynamic5QIDescriptor_t *toFill) {
   asn1cCalloc(toFill, tmp);