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