fix `add_assign` for `ProjectivePoint` (#543) #306

Job	Run time
Build starknet-ff features	1m 28s
Build starknet-curve features	1m 3s
Build starknet-crypto-codegen features	1m 2s
Build starknet-crypto features	1m 12s
Build starknet-core features	1m 33s
Build starknet-providers features	1m 44s
Build starknet-signers features	1m 16s
Build starknet-accounts features	1m 25s
Build starknet-contract features	1m 37s
Build starknet-macros features	1m 42s
Build starknet features	2m 13s
	16m 15s

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