link with rt to add clock-functions

CMakeLists.txt

@@ -217,7 +217,7 @@ if(EXTENSION_UDP)
 	list(APPEND server_sources examples/networklayer_udp.c)
 endif()
 add_executable(exampleServer ${server_sources} ${exported_headers} ${generated_headers})
-target_link_libraries(exampleServer open62541)
+target_link_libraries(exampleServer open62541 rt)
 if(WIN32)
     target_link_libraries(exampleServer ws2_32)
 endif(WIN32)