Add openssl to the required library list in pkg_config file

[accelerated]

Add openssl to the required library list in pkg_config file when building a static library. Otherwise pkg_config complains if CASS_USE_OPENSSL is enabled. Generated .pc file is:

prefix=/usr/local
exec_prefix=/usr/local
libdir=/usr/local/lib64
includedir=/usr/local/include/cassandra-datastax

Name: cassandra
Description: A C/C++ client driver for Apache Cassandra
Version: 2.13.0
Requires: libuv openssl
Libs: -L${libdir} -lcassandra_static -lstdc++
Cflags:
URL: https://github.com/datastax/cpp-driver/

[mikefero]

Thanks for the contribution.