[libsigcplusplus] trackable: Use emplace_back() instead of push_back().

[Murray Cumming <murrayc src gnome org>]

commit 562d2095e03664b63b4bfbbddda472d3d7373568
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Jul 21 08:18:50 2017 +0200

    trackable: Use emplace_back() instead of push_back().
    
    Found by clang-tidy.

 sigc++/trackable.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/trackable.cc b/sigc++/trackable.cc
index 8ee8ee4..fdff747 100644
--- a/sigc++/trackable.cc
+++ b/sigc++/trackable.cc
@@ -119,7 +119,7 @@ trackable_callback_list::add_callback(notifiable* data, func_destroy_notify func
   // I'd consider this a serious application bug, since the app is likely to segfault.
   // But then, how should we handle it? Throw an exception? Martin.
   if (!clearing_)
-    callbacks_.push_back(trackable_callback(data, func));
+    callbacks_.emplace_back(trackable_callback(data, func));
 }
 
 void