MXS-2483: Fix dcb.hh includes

The header depended on ssl.hh to include the OpenSSL headers even though
it used OpenSSL types. By fixing these dependencies the ssl.h header can
now freely include the rworker_local type which removes the need for the
hidden implementation of SSLProvider.

include/maxscale/dcb.hh

@@ -17,13 +17,15 @@
  */
 
 #include <maxscale/ccdefs.hh>
+
+#include <openssl/ssl.h>
 #include <netinet/in.h>
+
 #include <maxbase/poll.h>
 #include <maxscale/authenticator.hh>
 #include <maxscale/buffer.hh>
 #include <maxscale/modinfo.h>
 #include <maxscale/protocol.hh>
-#include <maxscale/ssl.hh>
 
 #include <memory>