fix `add_assign` for `ProjectivePoint` #304

Job	Run time
Build starknet-ff features	1m 24s
Build starknet-curve features	1m 4s
Build starknet-crypto-codegen features	1m 5s
Build starknet-crypto features	1m 3s
Build starknet-core features	1m 37s
Build starknet-providers features	1m 48s
Build starknet-signers features	1m 12s
Build starknet-accounts features	1m 43s
Build starknet-contract features	1m 34s
Build starknet-macros features	1m 6s
Build starknet features	2m 8s
	15m 44s

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