Rename the GOST engine

The GOST engine file was "libgost_engine.so".  Unfortunately, this
will fail to load with any OpenSSL version, as the derived engine ID
will be "gost_engine" (OpenSSL <1.1.0) or "libgost_engine" (OpenSSL
>=1.1.0), since the engine checks the ID internally against "gost".

To make the IDs compatible, make sure the file name will be "gost.so"