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/E1AP/e1ap.c

@@ -32,6 +32,7 @@
 #include "common/utils/LOG/log.h"
 #include "openair2/F1AP/f1ap_common.h"
 #include "e1ap_default_values.h"
+#include "gtp_itf.h"
 
 #define E1AP_NUM_MSG_HANDLERS 14
 typedef int (*e1ap_message_processing_t)(e1ap_upcp_inst_t *inst, const E1AP_E1AP_PDU_t *message_p);