Correct compilation when AP_RSSI_ENABLED is false #26925

Job	Run time
build (chibios, Durandal)	15m 48s
build (chibios, MatekF405)	15m 2s
build (chibios, KakuteF7)	13m 33s
build (chibios, MatekH743-bdshot)	16m 7s
build (chibios, Pixhawk1-1M)	13m 56s
build (chibios, MatekF405-CAN)	1m 49s
build (chibios, DrotekP3Pro)	1m 55s
build (chibios, Hitec-Airspeed)	4m 5s
build (chibios, f103-GPS)	4m 6s
build (disco, armhf)	14m 29s
	1h 40m 50s

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