[libsigcplusplus] temp_slot_list: Make constructor explicit.

[Murray Cumming <murrayc src gnome org>]

commit 1cd5f2b5d71e464f281726afe62b6e039400a5d1
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Apr 21 23:42:32 2016 +0200

    temp_slot_list: Make constructor explicit.

 sigc++/signal.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/signal.h b/sigc++/signal.h
index 9f2af2a..11ec37b 100644
--- a/sigc++/signal.h
+++ b/sigc++/signal.h
@@ -205,7 +205,7 @@ struct temp_slot_list
   using iterator = signal_impl::iterator_type;
   using const_iterator = signal_impl::const_iterator_type;
 
-  temp_slot_list(slot_list& slots) : slots_(slots)
+  explicit temp_slot_list(slot_list& slots) : slots_(slots)
   {
     placeholder = slots_.insert(slots_.end(), slot_base());
   }