This patch comes from upstream:
https://hg.savannah.gnu.org/hgweb/octave/rev/cdaa884568b1.

# HG changeset patch
# User Mike Miller <mtmiller@octave.org>
# Date 1527214835 25200
# Node ID cdaa884568b159549bd373f04386ff62417f6df9
# Parent  9e39a53b4e007d3f79f88b711ab9fa5f2f24fbc9
add Qt include needed to build against Qt 5.11 (bug #53978)

* settings-dialog.cc: Add missing include for <QButtonGroup> to fix build
failure with Qt 5.11.

diff --git a/libgui/src/settings-dialog.cc b/libgui/src/settings-dialog.cc
--- a/libgui/src/settings-dialog.cc
+++ b/libgui/src/settings-dialog.cc
@@ -34,6 +34,8 @@
 #include "workspace-model.h"
 #include "settings-dialog.h"
 #include "ui-settings-dialog.h"
+
+#include <QButtonGroup>
 #include <QDir>
 #include <QFileInfo>
 #include <QFileDialog>