RFemulator: initialize RNG when using noise device

After !3786, we always have to initialize the RNG when using random
numbers. This was not done for RFemulator and has been added here.

radio/emulator/rf_emulator.c

@@ -364,6 +364,8 @@ int device_init(openair0_device *device, openair0_config_t *openair0_cfg)
   device->trx_read_func = emulator_read;
 
   emulator_readconfig(emulator_state);
+  if (emulator_state->enable_noise)
+    randominit(); // for noise device
 
   return 0;
 }