Fix some namespace formatting

Why not. :)
Found by clang-tidy.

Change-Id: I99a9cf8efa71c446f5a1ae9c3285d96fe8152915
diff --git a/src/netconf_access.hpp b/src/netconf_access.hpp
index cee092f..27e54e2 100644
--- a/src/netconf_access.hpp
+++ b/src/netconf_access.hpp
@@ -16,11 +16,9 @@
  *     \brief Implementation of DatastoreAccess for accessing a NETCONF server
  */
 
-namespace libnetconf {
-namespace client {
+namespace libnetconf::client {
 class Session;
 }
-}
 
 class Schema;
 class YangSchema;