Ensure message-handler can be compiled without EthosU support

Change-Id: I1ae0b1fa6d3c559afd10772778c16002ae93cc63
diff --git a/applications/message_handler/message_handler.cpp b/applications/message_handler/message_handler.cpp
index 4b77389..6630c0a 100644
--- a/applications/message_handler/message_handler.cpp
+++ b/applications/message_handler/message_handler.cpp
@@ -513,6 +513,7 @@
         job.clean();
     }
 
+#if defined(ETHOSU)
     /*
      * Print PMU counters
      */
@@ -528,6 +529,7 @@
             printf("ethosu_pmu_cycle_cntr : %" PRIu64 " cycles\n", rsp.pmu_cycle_counter_count);
         }
     }
+#endif
 
     /*
      * Send inference response
@@ -699,6 +701,7 @@
 
 } // namespace MessageHandler
 
+#if defined(ETHOSU)
 extern "C" void ethosu_inference_begin(struct ethosu_driver *drv, void *userArg) {
     MessageHandler::InferenceHandler *self = static_cast<MessageHandler::InferenceHandler *>(userArg);
 
@@ -754,3 +757,4 @@
     // Disable PMU
     ETHOSU_PMU_Disable(drv);
 }
+#endif