AP_Terrain: Add const to locals #44284

Job	Run time
build (stm32f7, chibios, 10)	3m 39s
build (stm32h7, chibios, 10)	4m 1s
build (fmuv2-plane, chibios, 10)	1m 50s
build (periph-build, chibios, 10)	4m 3s
build (iofirmware, chibios, 10)	1m 35s
build (CubeOrange-bootloader, chibios, 10)	1m 7s
build (fmuv3-bootloader, chibios, 10)	50s
build (revo-bootloader, chibios, 10)	54s
build (stm32h7-debug, chibios, 10)	2m 24s
build (fmuv3, chibios, 10)	5m 24s
build (revo-mini, chibios, 10)	4m 6s
build (MatekF405-Wing, chibios, 10)	4m 52s
build (CubeOrange-ODID, chibios, 10)	2m 32s
build (CubeRedPrimary-bootloader, chibios, 10)	48s
build (configure-all, chibios, 10)	10m 50s
build (build-options-defaults-test, chibios, 10)	7m 28s
build (signing, chibios, 10)	2m 42s
build (CubeOrange-PPP, chibios, 10)	2m 2s
build (CubeRed-EKF2, chibios, 10)	2m 1s
build (SOHW, chibios, 10)	3m 38s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 12s
build (new-check, chibios, 10)	4m 20s
	1h 12m 18s

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