AP_Terrain: Add const to locals #41444

Job	Run time
build (base)	2m 15s
build (clang)	2m 0s
build-gcc-heli	2m 10s
autotest (sitltest-copter-tests1a)	7m 32s
autotest (sitltest-copter-tests1b)	9m 30s
autotest (sitltest-copter-tests1c)	11m 36s
autotest (sitltest-copter-tests1d)	9m 17s
autotest (sitltest-copter-tests1e)	9m 3s
autotest (sitltest-copter-tests2a)	8m 54s
autotest (sitltest-copter-tests2b)	28m 30s
autotest-heli (sitltest-heli)	9m 54s
	1h 40m 41s

Provide feedback