GCS_MAVLink: correct compilation when compass not enabled #34116

Job	Run time
build (stm32f7, chibios, 10)	4m 12s
build (stm32h7, chibios, 10)	3m 45s
build (fmuv2-plane, chibios, 10)	2m 14s
build (periph-build, chibios, 10)	5m 10s
build (iofirmware, chibios, 10)	1m 48s
build (CubeOrange-bootloader, chibios, 10)	1m 9s
build (fmuv3-bootloader, chibios, 10)	51s
build (revo-bootloader, chibios, 10)	52s
build (stm32h7-debug, chibios, 10)	2m 10s
build (fmuv3, chibios, 10)	5m 7s
build (revo-mini, chibios, 10)	3m 53s
build (MatekF405-Wing, chibios, 10)	4m 39s
build (CubeOrange-ODID, chibios, 10)	2m 47s
build (CubeRedPrimary-bootloader, chibios, 10)	52s
build (configure-all, chibios, 10)	9m 22s
build (build-options-defaults-test, chibios, 10)	6m 16s
build (signing, chibios, 10)	2m 40s
	57m 47s

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