Set OPENSSL_ROOT_DIR if it isn't set.

These days, the OpenSSL cmake package finder is capable of finding
a custom OpenSSL installation with no OPENSSL_ROOT_DIR defined, but
CMAKE_PREFIX_PATH defined.

However, gost-engine's CMakeLists.txt does use OPENSSL_ROOT_DIR, so
it needs it to be defined unconditionally.  That's arranged fairly
easily by assigning it the parent directory of OPENSSL_INCLUDE_DIR.