AP_Compass: fix example by instantiating serial_manager

libraries/AP_Compass/examples/AP_Compass_test/AP_Compass_test.cpp

@@ -35,6 +35,7 @@ public:
 static DummyVehicle vehicle;
 // create compass object
 static Compass compass;
+static AP_SerialManager serial_manager;
 
 uint32_t timer;