[glibmm] Examples: Make a single-parameter constructor explicit.

[Murray Cumming <murrayc src gnome org>]

commit b495ba6b0b1e51fc36d7aa97ed398a7ce2e542e3
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Dec 12 10:38:58 2016 +0100

    Examples: Make a single-parameter constructor explicit.
    
    Noticed by cppcheck.

 examples/child_watch/main.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/child_watch/main.cc b/examples/child_watch/main.cc
index 7738904..a995f5a 100644
--- a/examples/child_watch/main.cc
+++ b/examples/child_watch/main.cc
@@ -25,7 +25,7 @@ using namespace std;
 class ChildWatch : public sigc::trackable
 {
 public:
-  ChildWatch(const Glib::RefPtr<Glib::MainLoop>& mainLoop) : m_mainLoop(mainLoop) {}
+  explicit ChildWatch(const Glib::RefPtr<Glib::MainLoop>& mainLoop) : m_mainLoop(mainLoop) {}
 
   void on_child_exited(GPid pid, int status);
   void run(); // fork a child and call signal_child_watch