AP_Math: add missing include in vector2.h #647

Job	Run time
build (stm32f7, chibios, 10)	14m 18s
build (stm32h7, chibios, 10)	12m 58s
build (fmuv2-plane, chibios, 10)	5m 45s
build (periph-build, chibios, 10)	18m 41s
build (iofirmware, chibios, 10)	2m 30s
build (CubeOrange-bootloader, chibios, 10)	1m 28s
build (fmuv3-bootloader, chibios, 10)	1m 28s
build (revo-bootloader, chibios, 10)	1m 10s
build (stm32h7-debug, chibios, 10)	6m 58s
build (fmuv3, chibios, 10)	13m 17s
build (revo-mini, chibios, 10)	10m 44s
build (MatekF405-Wing, chibios, 10)	11m 25s
build (CubeOrange-ODID, chibios, 10)	8m 31s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 11s
build (configure-all, chibios, 10)	9m 31s
build (build-options-defaults-test, chibios, 10)	19m 39s
build (signing, chibios, 10)	9m 27s
build (CubeOrange-PPP, chibios, 10)	6m 53s
build (CubeOrange-SOHW, chibios, 10)	6m 49s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 23s
	2h 46m 6s

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