AP_Vehicle: fix compile when AP_EXTERNAL_CONTROL_ENABLED disabled #673

Job	Run time
build (stm32f7, chibios, 10)	16m 8s
build (stm32h7, chibios, 10)	14m 25s
build (fmuv2-plane, chibios, 10)	7m 52s
build (periph-build, chibios, 10)	14m 27s
build (iofirmware, chibios, 10)	4m 58s
build (CubeOrange-bootloader, chibios, 10)	3m 18s
build (fmuv3-bootloader, chibios, 10)	3m 32s
build (revo-bootloader, chibios, 10)	3m 31s
build (stm32h7-debug, chibios, 10)	9m 2s
build (fmuv3, chibios, 10)	15m 30s
build (revo-mini, chibios, 10)	12m 39s
build (MatekF405-Wing, chibios, 10)	13m 10s
build (CubeOrange-ODID, chibios, 10)	10m 28s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 39s
build (configure-all, chibios, 10)	13m 1s
build (build-options-defaults-test, chibios, 10)	28m 20s
build (signing, chibios, 10)	11m 26s
build (CubeOrange-PPP, chibios, 10)	8m 44s
build (SOHW, chibios, 10)	14m 34s
build (Pixhawk6X-PPPGW, chibios, 10)	4m 57s
build (new-check, chibios, 10)	15m 11s
	3h 48m 52s

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