AP_HAL_Linux: send warning to stderr

libraries/AP_HAL_Linux/Scheduler.cpp

@@ -66,7 +66,7 @@ void Scheduler::init_realtime()
 #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");
+        fprintf(stderr, "WARNING: not running as root. Will not use realtime scheduling\n");
         return;
     }
 #endif