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

executables/nr-uesoftmodem.c

@@ -59,6 +59,7 @@ unsigned short config_frames[4] = {2,9,11,13};
 
 #include "UTIL/OPT/opt.h"
 #include "enb_config.h"
+#include "pdcp.h"
 
 #include "intertask_interface.h"