GEODE-7182: fix a warning in TcpSslConn.cpp that prevents successful compilation on gcc 8.3 (#515)

diff --git a/cppcache/src/TcpSslConn.cpp b/cppcache/src/TcpSslConn.cpp
index b4b328e..22b59aa 100644
--- a/cppcache/src/TcpSslConn.cpp
+++ b/cppcache/src/TcpSslConn.cpp
@@ -57,7 +57,7 @@
   LOGDEBUG("Creating SSL socket stream");
   try {
     m_ssl = getSSLImpl(sock, m_pubkeyfile, m_privkeyfile);
-  } catch (std::exception e) {
+  } catch (std::exception& e) {
     throw SslException(e.what());
   }
 }