SITL: add some magnetometer offsets

this ensures the compass null_offset algorithm has some work to do in
autotest

libraries/Desktop/support/sitl_compass.cpp

@@ -16,6 +16,10 @@
 #include "desktop.h"
 #include "util.h"
 
+#define MAG_OFS_X 17.0
+#define MAG_OFS_Y 125.0
+#define MAG_OFS_Z -18.0
+
 /*
   given a magnetic heading, and roll, pitch, yaw values,
   calculate consistent magnetometer components
@@ -65,5 +69,5 @@ void sitl_update_compass(float heading, float roll, float pitch, float yaw)
 				    ToRad(roll),
 				    ToRad(pitch),
 				    ToRad(yaw));
-	compass.setHIL(m.x, m.y, m.z);
+	compass.setHIL(m.x - MAG_OFS_X, m.y - MAG_OFS_Y, m.z - MAG_OFS_Z);
 }