Merge #94: secp256k1: enable ellswift module

d132eec secp256k1: enable ellswift module (Marko Bencun)

Pull request description:

  When running `cargo tarpaulin` (for test coverage) in a project that depends on this crate, compilation fails with:

  ```
  = note: /usr/bin/ld: /rust-bitcoinconsensus/target/debug/build/bitcoinconsensus-6e1dd1af27ef0a27/out/libbitcoinconsensus.a(0ad13e9c6ad06456-pubkey.o): in function `EllSwiftPubKey::Decode() const':
            /rust-bitcoinconsensus/depend/bitcoin/src/pubkey.cpp:355: undefined reference to `secp256k1_ellswift_decode'
            collect2: error: ld returned 1 exit status

  ```

  I am not sure why it's not a problem when compiling using `cargo test`, but the code does reference this function, so the module should be enabled.

ACKs for top commit:
  apoelstra:
    ACK d132eec

Tree-SHA512: 0c6fd7f8c935f44144aea75688cb8e83d0b6ff260ff0b9843c7cd64ed6bb2ed9a1da420152db390dff1447a308d0217ea6b39e33129c914a5944a6a87691860c

build.rs

@@ -42,7 +42,9 @@ fn main() {
             .define("ECMULT_GEN_PREC_BITS", "4")
             .define("ENABLE_MODULE_SCHNORRSIG", "1")
             .define("ENABLE_MODULE_EXTRAKEYS", "1")
-            // Technically libconsensus doesn't require the recovery feautre, but `pubkey.cpp` does.
+            // Technically libconsensus doesn't require the ellswift and recovery features, but
+            // `pubkey.cpp` does.
+            .define("ENABLE_MODULE_ELLSWIFT", "1")
             .define("ENABLE_MODULE_RECOVERY", "1")
             .file("depend/bitcoin/src/secp256k1/src/precomputed_ecmult_gen.c")
             .file("depend/bitcoin/src/secp256k1/src/precomputed_ecmult.c")