Add bench_internal to gitignore.
Add bench_internal to gitignore.

shea256/secp256k1-zkp,libbitcoin/secp256k1,chfast/secp256k1,evoskuil/secp256k1,pmienk/secp256k1,chfast/secp256k1,afk11/secp256k1,cddjr/BitcoinUnlimited,voisine/secp256k1,martindale/secp256k1,Justaphf/BitcoinUnlimited,digitalbitbox/secp256k1,digitalbitbox/secp256k1,instagibbs/secp256k1,Bitcoin-com/BUcash,sipa/secp256k1-zkp,digitalbitbox/secp256k1,digitalbitbox/secp256k1,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,martindale/secp256k1,Bitcoin-com/BUcash,libmetrocoin/secp256k1,ElementsProject/secp256k1-zkp,BTCfork/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,voisine/secp256k1,shea256/secp256k1-zkp,afk11/secp256k1,randy-waterhouse/secp256k1,voisine/secp256k1,marcusdiaz/BitcoinUnlimited,bitcoin/secp256k1,rustyrussell/secp256k1,cddjr/BitcoinUnlimited,shea256/secp256k1-zkp,afk11/secp256k1,marcusdiaz/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,chfast/secp256k1,evoskuil/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,digitalbitbox/secp256k1,Gustav-Simonsson/secp256k1,vlajos/secp256k1,sipa/secp256k1,01BTC10/secp256k1,kleetus/secp256k1,Bitcoin-com/BUcash,marcusdiaz/BitcoinUnlimited,shea256/secp256k1-zkp,GSongHashrate/secp256k1,martindale/secp256k1,BitcoinUnlimited/BitcoinUnlimited,peterdettman/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Gustav-Simonsson/secp256k1,kleetus/secp256k1,Bitcoin-com/BUcash,pmienk/secp256k1,chfast/secp256k1,bitcoin/secp256k1,marcusdiaz/BitcoinUnlimited,voisine/secp256k1,krzysztofwos/BitcoinUnlimited,bitcoin-core/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,BitcoinUnlimited/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,sipa/secp256k1-zkp,bitcoin/secp256k1,kleetus/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,instagibbs/secp256k1,ChrisXin/secp256k1,01BTC10/secp256k1,randy-waterhouse/secp256k1,libbitcoin/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,rustyrussell/secp256k1,marcusdiaz/BitcoinUnlimited,bitcoin/secp256k1,01BTC10/secp256k1,sipa/secp256k1,krzysztofwos/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,libmetrocoin/secp256k1,sipa/secp256k1,bitcoin/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ElementsProject/secp256k1-zkp,chfast/secp256k1,rustyrussell/secp256k1,libbitcoin/secp256k1,pmienk/secp256k1,Bitcoin-com/BUcash,apoelstra/secp256k1,vlajos/secp256k1,ChrisXin/secp256k1,Bitcoin-com/BUcash,libmetrocoin/secp256k1,GSongHashrate/secp256k1,cddjr/BitcoinUnlimited,afk11/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,instagibbs/secp256k1,kleetus/secp256k1,krzysztofwos/BitcoinUnlimited,01BTC10/secp256k1,ChrisXin/secp256k1,vlajos/secp256k1,BitcoinUnlimited/BitcoinUnlimited,Gustav-Simonsson/secp256k1,GSongHashrate/secp256k1,sipa/secp256k1,instagibbs/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,kleetus/secp256k1,pmienk/secp256k1,krzysztofwos/BitcoinUnlimited,peterdettman/secp256k1,bitcoin-core/secp256k1,ChrisXin/secp256k1,randy-waterhouse/secp256k1,apoelstra/secp256k1,rustyrussell/secp256k1,krzysztofwos/BitcoinUnlimited,martindale/secp256k1,marcusdiaz/BitcoinUnlimited,sipa/secp256k1-zkp,martindale/secp256k1-zkp,apoelstra/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,cddjr/BitcoinUnlimited,apoelstra/secp256k1,Gustav-Simonsson/secp256k1,sipa/secp256k1-zkp,evoskuil/secp256k1,instagibbs/secp256k1,martindale/secp256k1-zkp,randy-waterhouse/secp256k1,vlajos/secp256k1,apoelstra/secp256k1,libmetrocoin/secp256k1,evoskuil/secp256k1,sipa/secp256k1-zkp,cddjr/BitcoinUnlimited,afk11/secp256k1,martindale/secp256k1-zkp,cddjr/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,evoskuil/secp256k1,GSongHashrate/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,martindale/secp256k1-zkp,libbitcoin/secp256k1,voisine/secp256k1,libbitcoin/secp256k1,pmienk/secp256k1