don't print info message about zero mag neutrals if USE_MAGNETOMETER is false

conf/airframes/examples/bixler_lisa_m_2.xml

@@ -15,6 +15,7 @@
     <target name="sim" 			board="pc"/>
 
     <define name="ALT_KALMAN"/>
+    <define name="USE_MAGNETOMETER" value="FALSE"/>
 
     <subsystem name="radio_control" type="ppm"/>
     <subsystem name="telemetry" type="transparent"/>

sw/airborne/subsystems/imu.c

@@ -39,7 +39,9 @@ void imu_init(void) {
 #if defined IMU_MAG_X_NEUTRAL && defined IMU_MAG_Y_NEUTRAL && defined IMU_MAG_Z_NEUTRAL
   VECT3_ASSIGN(imu.mag_neutral,   IMU_MAG_X_NEUTRAL,   IMU_MAG_Y_NEUTRAL,   IMU_MAG_Z_NEUTRAL);
 #else
+#if USE_MAGNETOMETER
 #pragma message "Info: Magnetomter neutrals are set to zero!"
+#endif
   INT_VECT3_ZERO(imu.mag_neutral);
 #endif