[libsigcplusplus] signal_impl_exec_holder: Make a constructor explicit.

[Murray Cumming <murrayc src gnome org>]

commit 3ff8ca086fdb327c3f87a8d61a19a5cd8e826926
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Jul 21 08:29:22 2017 +0200

    signal_impl_exec_holder: Make a constructor explicit.
    
    Because it has only one parameter.
    Noticed by clang-tidy.

 sigc++/signal_base.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/signal_base.h b/sigc++/signal_base.h
index 514ddfe..3bbcd4f 100644
--- a/sigc++/signal_base.h
+++ b/sigc++/signal_base.h
@@ -178,7 +178,7 @@ struct SIGC_API signal_impl_exec_holder
   /** Increments the execution counter of the parent sigc::signal_impl object.
    * @param sig The parent sigc::signal_impl object.
    */
-  inline signal_impl_exec_holder(signal_impl* sig) noexcept
+  inline explicit signal_impl_exec_holder(signal_impl* sig) noexcept
   : sig_(sig)
   {
     sig_->reference_exec();