AP_HAL_Linux: do not attempt to run examples realtime unless root

libraries/AP_HAL_Linux/Scheduler.cpp

@@ -63,6 +63,13 @@ void Scheduler::init_realtime()
     // we don't run Replay in real-time...
     return;
 #endif
+#if APM_BUILD_TYPE(APM_BUILD_UNKNOWN)
+    // we opportunistically run examples/tools in realtime
+    if (geteuid() != 0) {
+        printf("WARNING: not running as root. Will not use realtime scheduling\n");
+        return;
+    }
+#endif
 
     mlockall(MCL_CURRENT|MCL_FUTURE);