desktop: make AP_InertialSensor pure virtual

fixed a link error in Desktop build

thanks to Pat for the C++ foo

libraries/AP_InertialSensor/AP_InertialSensor.h

@@ -12,7 +12,7 @@ class AP_InertialSensor
   public:
   AP_InertialSensor() {}
 
-  virtual void init( AP_PeriodicProcess * scheduler );
+  virtual void init( AP_PeriodicProcess * scheduler ) = 0;
 
   /* Update the sensor data, so that getters are nonblocking.
    * Returns a bool of whether data was updated or not.