fix USE_MME compilation flag

We should use ENABLE_USE_MME, not USE_MME.
This code is useless anyway (I think, not sure, that's why I
leave it here), because the other threads wait for 'sync_var'
to become >=0.

targets/RT/USER/lte-ue.c

@@ -198,14 +198,14 @@ void init_UE(int nb_inst) {
   }
 
   printf("UE threads created\n");
-#ifdef USE_MME
-  
+#if 0
+#if defined(ENABLE_USE_MME)
+  extern volatile int start_UE;
   while (start_UE == 0) {
     sleep(1);
   }
-  
 #endif
-  
+#endif
 }
 
 /*!