Fix build follow up

NathanW2 · NathanW2 · commit ee7ca0a5260b · 2018-10-08T10:40:02.000+10:00
diff --git a/src/core/qgsspatialindexkdbush.h b/src/core/qgsspatialindexkdbush.h
@@ -29,6 +29,7 @@ class QgsRectangle;
 #include "qgspointxy.h"
 #include <memory>
 #include <QList>
+#include <functional>
 
 /**
  * \class QgsSpatialIndexKDBush
