Rover: use new scheduler API

APMrover2/failsafe.pde

@@ -13,11 +13,12 @@
   this failsafe_check function is called from the core timer interrupt
   at 1kHz.
  */
-void failsafe_check(uint32_t tnow)
+void failsafe_check(void *arg)
 {
     static uint16_t last_mainLoop_count;
     static uint32_t last_timestamp;
     static bool in_failsafe;
+    uint32_t tnow = hal.scheduler->micros();
 
     if (mainLoop_count != last_mainLoop_count) {
         // the main loop is running, all is OK