Fix linking OpenSSL (branch release/1.8.x)

[johndebord]

Fixes this potential linker error when building eosio.contracts on MacOS:

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[100%] Linking CXX executable unit_test
Undefined symbols for architecture x86_64:
  "_BN_is_negative", referenced from:
      fc::bigint::is_negative() const in libfc.a(bigint.cpp.o)
      fc::bigint::to_int64() const in libfc.a(bigint.cpp.o)
  "_BN_is_zero", referenced from:
      fc::bigint::operator bool() const in libfc.a(bigint.cpp.o)
  "_ECDSA_SIG_get0", referenced from:
      fc::crypto::r1::ECDSA_SIG_recover_key_GFp(ec_key_st*, ECDSA_SIG_st*, unsigned char const*, int, int, int) in libfc.a(elliptic_r1.cpp.o)
      fc::crypto::r1::signature_from_ecdsa(ec_key_st const*, fc::array<char, 33ul> const&, fc::ecdsa_sig&, fc::sha256 const&) in libfc.a(elliptic_r1.cpp.o)
  "_ECDSA_SIG_set0", referenced from:
      fc::crypto::r1::signature_from_ecdsa(ec_key_st const*, fc::array<char, 33ul> const&, fc::ecdsa_sig&, fc::sha256 const&) in libfc.a(elliptic_r1.cpp.o)
      fc::crypto::r1::public_key::public_key(fc::array<unsigned char, 65ul> const&, fc::sha256 const&, bool) in libfc.a(elliptic_r1.cpp.o)
      fc::crypto::webauthn::public_key::public_key(fc::crypto::webauthn::signature const&, fc::sha256 const&, bool) in libfc.a(elliptic_webauthn.cpp.o)
  "_OPENSSL_init_crypto", referenced from:
      fc::openssl_scope::openssl_scope() in libfc.a(openssl.cpp.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[5]: *** [CMakeFiles/unit_test.dir/build.make:166: unit_test] Error 1
make[4]: *** [CMakeFiles/Makefile2:644: CMakeFiles/unit_test.dir/all] Error 2
make[3]: *** [Makefile:95: all] Error 2
make[2]: *** [CMakeFiles/contracts_unit_tests.dir/build.make:113: contracts_unit_tests-prefix/src/contracts_unit_tests-stamp/contracts_unit_tests-build] Error 2
make[1]: *** [CMakeFiles/Makefile2:78: CMakeFiles/contracts_unit_tests.dir/all] Error 2
make: *** [Makefile:84: all] Error 2
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