Include C dep "github.com/ethereum/go-ethereum/crypto/secp256k1"

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Project relies on C dependencies which are not provided by our dependency manager dep. Need to find a way to ensure it's included as part of dep ensure or by some other means.

Attempting to run tests today produces an error:

# github.com/OpenBazaar/multiwallet/vendor/github.com/ethereum/go-ethereum/crypto/secp256k1
vendor/github.com/ethereum/go-ethereum/crypto/secp256k1/curve.go:42:10: fatal error: 'libsecp256k1/include/secp256k1.h' file not found
#include "libsecp256k1/include/secp256k1.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

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This has been fixed and merged.