[libsigcplusplus/libsigc++-2-8] reference_wrapper: Include <functional>.

[Murray Cumming <murrayc src gnome org>]

commit c21aba8a028f0b9b33bc0f0254f80e897738e911
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Mar 8 11:17:47 2016 +0100

    reference_wrapper: Include <functional>.

 sigc++/reference_wrapper.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/reference_wrapper.h b/sigc++/reference_wrapper.h
index 35b2ab6..c04fa4d 100644
--- a/sigc++/reference_wrapper.h
+++ b/sigc++/reference_wrapper.h
@@ -19,6 +19,8 @@
 #ifndef _SIGC_REFERENCE_WRAPPER_H_
 #define _SIGC_REFERENCE_WRAPPER_H_
 
+#include <functional> // For std::reference_wrapper.
+
 namespace sigc {
 
 #ifndef SIGCXX_DISABLE_DEPRECATED