wasm secp256k1 sig verification is missing

[tzemanovic]

Trying to use secp256k1 signatures fails verification in WASM VPs with a runtime error. This is because the fn verify_signature has conditional compilation for feature = "secp256k1-sign-verify", which controls whether the hmac feature is enabled for libsecp256k1 (https://github.com/paritytech/libsecp256k1). However, hmac is only needed for creating signatures not for verifying them, so we can enable these. We should make sure to have a test case for tx signed with a secp256k1 key