fix g++ build

tools/travis_linux_script.sh

@@ -56,7 +56,7 @@ echo "Building the C++ example"
 mkdir -p build && cd build
 cp ../../open62541.* .
 gcc -std=c99 -c open62541.c
-g++-4.8 ../examples/server.cpp -I./ open62541.o -o cpp-server
+g++-4.8 ../examples/server.cpp -I./ open62541.o -lrt -o cpp-server
 cd .. && rm build -rf 
 
 echo "Compile multithreaded version"