AP_AHRS: examples: correct AHRS_Test

This is required so gcs() returns an object

libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.cpp

@@ -6,6 +6,7 @@
 #include <AP_AHRS/AP_AHRS.h>
 #include <AP_HAL/AP_HAL.h>
 #include <AP_BoardConfig/AP_BoardConfig.h>
+#include <GCS_MAVLink/GCS.h>
 
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
@@ -95,4 +96,6 @@ void loop(void)
     }
 }
 
+GCS _gcs;
+
 AP_HAL_MAIN();