Add missing <functional> includes (#5630)

src/core/mergedproxymodel.cpp

@@ -23,6 +23,7 @@
 
 #include <QStringList>
 
+#include <functional>
 #include <limits>
 
 // boost::multi_index still relies on these being in the global namespace.

src/devices/giolister.cpp

@@ -17,6 +17,7 @@
 
 #include "config.h"
 
+#include <functional>
 #include <memory>
 
 #include <QFile>

src/library/groupbydialog.cpp

@@ -20,6 +20,8 @@
 
 #include <QPushButton>
 
+#include <functional>
+
 // boost::multi_index still relies on these being in the global namespace.
 using std::placeholders::_1;
 using std::placeholders::_2;