AP_Compass - fixed compile warning

libraries/AP_Compass/Compass.cpp

@@ -15,12 +15,12 @@ const AP_Param::GroupInfo Compass::var_info[] PROGMEM = {
 // their values.
 //
 Compass::Compass(void) :
+	product_id(AP_COMPASS_TYPE_UNKNOWN),
     _declination		(0.0),
     _learn(1),
     _use_for_yaw(1),
     _null_enable(false),
-    _null_init_done(false),
-	product_id(AP_COMPASS_TYPE_UNKNOWN)
+    _null_init_done(false)
 {
     // Default the orientation matrix to none - will be overridden at group load time
     // if an orientation has previously been saved.