Add string.h include to ecmult_impl
Add string.h include to ecmult_impl

`memcpy` and `memset` are used, so include the appropriate header
for the declaration.

peterdettman/secp256k1,libbitcoin/secp256k1,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,chfast/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,libbitcoin/secp256k1,evoskuil/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,sipa/secp256k1-zkp,cddjr/BitcoinUnlimited,bitcoin/secp256k1,evoskuil/secp256k1,afk11/secp256k1,BitcoinUnlimited/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,libbitcoin/secp256k1,BitcoinUnlimited/BitcoinUnlimited,cddjr/BitcoinUnlimited,afk11/secp256k1,marcusdiaz/BitcoinUnlimited,digitalbitbox/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,apoelstra/secp256k1,bitcoin/secp256k1,pmienk/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,evoskuil/secp256k1,marcusdiaz/BitcoinUnlimited,voisine/secp256k1,sipa/secp256k1-zkp,BitcoinUnlimited/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,apoelstra/secp256k1,Justaphf/BitcoinUnlimited,pmienk/secp256k1,sipa/secp256k1-zkp,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,pmienk/secp256k1,digitalbitbox/secp256k1,BitcoinUnlimited/BitcoinUnlimited,Bitcoin-com/BUcash,krzysztofwos/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,apoelstra/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1,cddjr/BitcoinUnlimited,afk11/secp256k1,Justaphf/BitcoinUnlimited,Bitcoin-com/BUcash,chfast/secp256k1,Justaphf/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,ElementsProject/secp256k1-zkp,BitcoinUnlimited/BitcoinUnlimited,sipa/secp256k1-zkp,evoskuil/secp256k1,cddjr/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,libbitcoin/secp256k1,chfast/secp256k1,bitcoin/secp256k1,chfast/secp256k1,cddjr/BitcoinUnlimited,Justaphf/BitcoinUnlimited,libbitcoin/secp256k1,evoskuil/secp256k1,digitalbitbox/secp256k1,digitalbitbox/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,voisine/secp256k1,chfast/secp256k1,bitcoin-core/secp256k1,Bitcoin-com/BUcash,BTCfork/hardfork_prototype_1_mvf-bu,bitcoin/secp256k1,Bitcoin-com/BUcash,afk11/secp256k1,krzysztofwos/BitcoinUnlimited,voisine/secp256k1,Bitcoin-com/BUcash,BTCfork/hardfork_prototype_1_mvf-bu,digitalbitbox/secp256k1,krzysztofwos/BitcoinUnlimited,Justaphf/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,voisine/secp256k1,pmienk/secp256k1,pmienk/secp256k1,marcusdiaz/BitcoinUnlimited,peterdettman/secp256k1,apoelstra/secp256k1,bitcoin-core/secp256k1,sipa/secp256k1-zkp,bitcoin/secp256k1,krzysztofwos/BitcoinUnlimited,apoelstra/secp256k1,voisine/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu