pingus: fix build failure

Closes #10873.

Signed-off-by: Enno Boland <gottox@voidlinux.eu>

srcpkgs/pingus/patches/missing-header.patch

@@ -0,0 +1,22 @@
+From df6e2f445d3e2925a94d22faeb17be9444513e92 Mon Sep 17 00:00:00 2001
+From: Jonathan Wakely <jwakely@redhat.com>
+Date: Mon, 30 Jan 2017 15:41:53 +0000
+Subject: [PATCH] Add missing header for std::function and std::bind
+
+---
+ src/pingus/screens/demo_session.cpp | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/src/pingus/screens/demo_session.cpp b/src/pingus/screens/demo_session.cpp
+index e3ec49a2c..f887e50f0 100644
+--- src/pingus/screens/demo_session.cpp
++++ src/pingus/screens/demo_session.cpp
+@@ -18,6 +18,7 @@
+ 
+ #include <algorithm>
+ #include <iostream>
++#include <functional>
+ 
+ #include "engine/gui/gui_manager.hpp"
+ #include "engine/gui/surface_button.hpp"
+