Function secp256k1_schnorrsig_sign_custom
pub unsafe extern "C" fn secp256k1_schnorrsig_sign_custom(
cx: *const Context,
sig: *mut u8,
msg: *const u8,
msg_len: usize,
keypair: *const KeyPair,
extra_params: *const SchnorrSigExtraParams,
) -> i32