AP_ExternalAHRS: Fix Build Option for InertialLabs #38912

Job	Run time
build (base)	4m 26s
build (base)	4m 26s
build-gcc-heli	4m 16s
build-gcc-heli	4m 16s
build (clang)	4m 8s
build (clang)	4m 8s
autotest (sitltest-copter-tests1a)	7m 28s
autotest (sitltest-copter-tests1a)	7m 28s
autotest (sitltest-copter-tests1b)	12m 36s
autotest (sitltest-copter-tests1b)	12m 36s
autotest (sitltest-copter-tests1c)	11m 35s
autotest (sitltest-copter-tests1c)	11m 35s
autotest (sitltest-copter-tests1d)	9m 24s
autotest (sitltest-copter-tests1d)	9m 1s
autotest (sitltest-copter-tests1e)	9m 47s
autotest (sitltest-copter-tests1e)	9m 47s
autotest (sitltest-copter-tests2a)	8m 56s
autotest (sitltest-copter-tests2a)	8m 56s
autotest (sitltest-copter-tests2b)	29m 3s
autotest (sitltest-copter-tests2b)	29m 3s
autotest-heli (sitltest-heli)	10m 23s
autotest-heli (sitltest-heli)	10m 23s
	3h 43m 41s

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