As per title, this is required to build tests when openssl is in a custom location.
Fixes the following error:
```
[257/259] Building CXX object src/test/CMakeFiles/test_bitcoin.dir/__/wallet/test/crypto_tests.cpp.o
FAILED: src/test/CMakeFiles/test_bitcoin.dir/__/wallet/test/crypto_tests.cpp.o
ccache /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DBOOST_TEST_DYN_LINK -DHAVE_CONFIG_H -DLEVELDB_ATOMIC_PRESENT -DLEVELDB_PLATFORM_POSIX -DOS_MACOSX -Isrc/test -isystem /usr/local/include -I../src/univalue/include -I../src/. -Isrc -Isrc/crypto/.. -I../src/secp256k1/include -I../src/leveldb/include -Wall -Wextra -Wformat -Wvla -Wformat-security -Wcast-align -Wunused-parameter -Wno-unused-parameter -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -fvisibility=hidden -std=gnu++11 -MD -MT src/test/CMakeFiles/test_bitcoin.dir/__/wallet/test/crypto_tests.cpp.o -MF src/test/CMakeFiles/test_bitcoin.dir/__/wallet/test/crypto_tests.cpp.o.d -o src/test/CMakeFiles/test_bitcoin.dir/__/wallet/test/crypto_tests.cpp.o -c ../src/wallet/test/crypto_tests.cpp
../src/wallet/test/crypto_tests.cpp:12:10: fatal error: 'openssl/aes.h' file not found
^~~~~~~~~~~~~~~
1 error generated.
```