AP_HAL_Empty: add check owner method

libraries/AP_HAL_Empty/Semaphores.h

@@ -8,6 +8,7 @@ public:
     bool give() override;
     bool take(uint32_t timeout_ms) override;
     bool take_nonblocking() override;
+    bool check_owner() { return true; }
 private:
     bool _taken;
 };