AP_RCProtocol: get rid of compiler warning from clang about unknown w… #28

Job	Run time
build (stm32f7, chibios, 10)	6m 6s
build (stm32h7, chibios, 10)	5m 46s
build (fmuv2-plane, chibios, 10)	4m 6s
build (periph-build, chibios, 10)	5m 43s
build (iofirmware, chibios, 10)	3m 41s
build (CubeOrange-bootloader, chibios, 10)	2m 55s
build (fmuv3-bootloader, chibios, 10)	3m 14s
build (revo-bootloader, chibios, 10)	2m 57s
build (stm32h7-debug, chibios, 10)	4m 41s
build (fmuv3, chibios, 10)	8m 0s
build (revo-mini, chibios, 10)	6m 37s
build (MatekF405-Wing, chibios, 10)	7m 1s
build (CubeOrange-ODID, chibios, 10)	5m 3s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 20s
build (configure-all, chibios, 10)	13m 25s
build (build-options-defaults-test, chibios, 10)	9m 33s
build (signing, chibios, 10)	5m 4s
build (CubeOrange-PPP, chibios, 10)	4m 43s
build (CubeRed-EKF2, chibios, 10)	4m 24s
build (SOHW, chibios, 10)	5m 57s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 38s
build (new-check, chibios, 10)	6m 54s
	2h 2m 48s

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