[glibmm] Examples: Network: Small const improvement.

[Murray Cumming <murrayc src gnome org>]

commit 247b21d3da677f060514ba8b0ad7c500016edcb7
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Dec 12 10:26:01 2016 +0100

    Examples: Network: Small const improvement.
    
    Noticed by cppcheck.

 examples/network/socket-client.cc |    2 +-
 examples/network/socket-server.cc |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/examples/network/socket-client.cc b/examples/network/socket-client.cc
index c364d9b..dcb8e90 100644
--- a/examples/network/socket-client.cc
+++ b/examples/network/socket-client.cc
@@ -128,7 +128,7 @@ cancel_thread(Glib::RefPtr<Gio::Cancellable> cancellable)
 class JoinAndDelete
 {
 public:
-  void operator()(std::thread* thread)
+  void operator()(std::thread* thread) const
   {
     stop_thread = true;
     cond_thread.notify_all();
diff --git a/examples/network/socket-server.cc b/examples/network/socket-server.cc
index fb9d6ea..5f25442 100644
--- a/examples/network/socket-server.cc
+++ b/examples/network/socket-server.cc
@@ -134,7 +134,7 @@ cancel_thread(Glib::RefPtr<Gio::Cancellable> cancellable)
 class JoinAndDelete
 {
 public:
-  void operator()(std::thread* thread)
+  void operator()(std::thread* thread) const
   {
     stop_thread = true;
     cond_thread.notify_all();