[libsigcplusplus] track_obj_functor: Make the constructor explicit.

[Murray Cumming <murrayc src gnome org>]

commit f7c2a73eb240ee4e9d88047cbd9d82088e835932
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 21 14:53:59 2017 +0200

    track_obj_functor: Make the constructor explicit.
    
    Because it can take just one argument.
    Noticed by clang-tidy.

 sigc++/adaptors/track_obj.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/adaptors/track_obj.h b/sigc++/adaptors/track_obj.h
index 9256011..4419ffb 100644
--- a/sigc++/adaptors/track_obj.h
+++ b/sigc++/adaptors/track_obj.h
@@ -75,7 +75,7 @@ public:
    * @param func Functor.
    * @param obj Trackable objects.
    */
-  track_obj_functor(const T_functor& func, const T_obj&... obj)
+  explicit track_obj_functor(const T_functor& func, const T_obj&... obj)
   : adapts<T_functor>(func), obj_(obj...)
   {
   }