FEATURE Add NETCONF time function

Add implementation of public functions to work with timestamps in the
NETCONF format.
diff --git a/nc_client.h b/nc_client.h
index ad261af..d055455 100644
--- a/nc_client.h
+++ b/nc_client.h
@@ -23,6 +23,7 @@
 #ifndef NC_CLIENT_H_
 #define NC_CLIENT_H_
 
+#include <libnetconf2/netconf.h>
 #include <libnetconf2/log.h>
 
 #endif /* NC_CLIENT_H_ */