AP_Periph: add missing AP_AHRS include #28

Job	Run time
build (stm32f7, chibios, 10)	14m 12s
build (stm32h7, chibios, 10)	12m 44s
build (fmuv2-plane, chibios, 10)	5m 45s
build (periph-build, chibios, 10)	19m 41s
build (iofirmware, chibios, 10)	2m 51s
build (CubeOrange-bootloader, chibios, 10)	1m 30s
build (fmuv3-bootloader, chibios, 10)	1m 25s
build (revo-bootloader, chibios, 10)	1m 28s
build (stm32h7-debug, chibios, 10)	6m 57s
build (fmuv3, chibios, 10)	13m 39s
build (revo-mini, chibios, 10)	10m 31s
build (MatekF405-Wing, chibios, 10)	11m 39s
build (CubeOrange-ODID, chibios, 10)	8m 28s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 32s
build (configure-all, chibios, 10)	9m 57s
build (build-options-defaults-test, chibios, 10)	20m 51s
build (signing, chibios, 10)	10m 38s
build (CubeOrange-PPP, chibios, 10)	7m 12s
build (CubeOrange-SOHW, chibios, 10)	6m 55s
build (Pixhawk6X-PPPGW, chibios, 10)	2m 59s
	2h 50m 54s

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