Fix compilation with gcc-4.9

Looks like one needs to include <functional> if one uses std::function.

Compilation verified with:
* gcc-4.8
* gcc-4.9
* clang-3.2

effects/effect_builtins.h

@@ -22,6 +22,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <kwineffects_export.h>
 #include <QStringList>
 #include <QUrl>
+#include <functional>
 
 namespace KWin
 {

toplevel.h

@@ -40,6 +40,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <fixx11h.h>
 // system
 #include <assert.h>
+// c++
+#include <functional>
 
 namespace KWin
 {