AP_HAL_VRBRAIN: make in_main_thread const and override

libraries/AP_HAL_VRBRAIN/Scheduler.cpp

@@ -370,7 +370,7 @@ void *VRBRAINScheduler::_storage_thread(void *arg)
     return nullptr;
 }
 
-bool VRBRAINScheduler::in_main_thread()
+bool VRBRAINScheduler::in_main_thread() const
 {
     return getpid() == _main_task_pid;
 }

libraries/AP_HAL_VRBRAIN/Scheduler.h

@@ -55,6 +55,7 @@ public:
     void     resume_timer_procs();
     void     reboot(bool hold_in_bootloader);
 
+    bool     in_main_thread() const override;
     void     system_initialized();
     void     hal_initialized() { _hal_initialized = true; }