DolphinQt: Explicitly include <QActionGroup> where applicable

An indirect inclusion scenario that breaks on Qt 6.0

Source/Core/DolphinQt/Debugger/RegisterWidget.cpp

@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include <QActionGroup>
 #include <QHeaderView>
 #include <QMenu>
 #include <QTableWidget>

Source/Core/DolphinQt/MenuBar.cpp

@@ -7,6 +7,7 @@
 #include <cinttypes>
 
 #include <QAction>
+#include <QActionGroup>
 #include <QDesktopServices>
 #include <QFileDialog>
 #include <QFontDialog>

Source/Core/DolphinQt/NetPlay/NetPlayDialog.cpp

@@ -5,6 +5,7 @@
 #include "DolphinQt/NetPlay/NetPlayDialog.h"
 
 #include <QAction>
+#include <QActionGroup>
 #include <QApplication>
 #include <QClipboard>
 #include <QComboBox>