[libsigcplusplus] slot_do_bind/unbind: Make constructors explicit.

[Murray Cumming <murrayc src gnome org>]

commit 0d038ec77fe6e1f19e140e89a43f9fe9ecbe2e02
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 17:09:08 2016 +0200

    slot_do_bind/unbind: Make constructors explicit.
    
    Found by cppcheck.

 sigc++/functors/slot_base.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/sigc++/functors/slot_base.h b/sigc++/functors/slot_base.h
index 353ec57..56cabc8 100644
--- a/sigc++/functors/slot_base.h
+++ b/sigc++/functors/slot_base.h
@@ -164,7 +164,7 @@ struct SIGC_API slot_do_bind
   /** Construct a slot_do_bind functor.
    * @param rep The slot_rep object trackables should notify on destruction.
    */
-  inline slot_do_bind(slot_rep* rep) noexcept : rep_(rep) {}
+  inline explicit slot_do_bind(slot_rep* rep) noexcept : rep_(rep) {}
 
   /** Adds a dependency to @p t.
    * @param t The trackable object to add a callback to.
@@ -184,7 +184,7 @@ struct SIGC_API slot_do_unbind
   /** Construct a slot_do_unbind functor.
    * @param rep The slot_rep object trackables don't need to notify on destruction any more.
    */
-  inline slot_do_unbind(slot_rep* rep) noexcept : rep_(rep) {}
+  inline explicit slot_do_unbind(slot_rep* rep) noexcept : rep_(rep) {}
 
   /** Removes a dependency from @p t.
    * @param t The trackable object to remove the callback from.