gitmodules: update mavlink submodule URL to ArduPilot repository #246

Job	Run time
build (stm32h7, chibios, 10)	12m 33s
build (stm32f7, chibios, 10)	13m 31s
build (fmuv2-plane, chibios, 10)	5m 27s
build (periph-build, chibios, 10)	17m 55s
build (CubeOrange-bootloader, chibios, 10)	1m 4s
build (iofirmware, chibios, 10)	2m 12s
build (fmuv3-bootloader, chibios, 10)	1m 7s
build (stm32h7-debug, chibios, 10)	6m 48s
build (fmuv3, chibios, 10)	13m 7s
build (revo-bootloader, chibios, 10)	1m 1s
build (CubeOrange-ODID, chibios, 10)	7m 53s
build (revo-mini, chibios, 10)	10m 4s
build (build-options-defaults-test, chibios, 10)	19m 20s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 24s
build (MatekF405-Wing, chibios, 10)	10m 49s
build (signing, chibios, 10)	9m 4s
build (CubeOrange-SOHW, chibios, 10)	7m 6s
build (configure-all, chibios, 10)	9m 17s
build (CubeOrange-PPP, chibios, 10)	6m 24s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 1s
	2h 39m 7s

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