[LyX/master] Fix for Qt < 5.11

[Juergen Spitzmueller]

commit 9aad98836d12073e5a27993e4af899339088aa55
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Sun Mar 7 14:33:13 2021 +0100

    Fix for Qt < 5.11
---
 src/frontends/qt/GuiView.cpp |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/src/frontends/qt/GuiView.cpp b/src/frontends/qt/GuiView.cpp
index 1d779fc..e9ab64b 100644
--- a/src/frontends/qt/GuiView.cpp
+++ b/src/frontends/qt/GuiView.cpp
@@ -630,7 +630,11 @@ GuiView::GuiView(int id)
 	QFontMetrics const fm(statusBar()->fontMetrics());
 
 	QSlider * zoomslider = new QSlider(Qt::Horizontal, statusBar());
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 11, 0))
 	zoomslider->setFixedWidth(fm.horizontalAdvance('x') * 15);
+#else
+	zoomslider->setFixedWidth(fm.width('x') * 15);
+#endif
 	// Make the defaultZoom center
 	zoomslider->setRange(10, (lyxrc.defaultZoom * 2) - 10);
 	zoomslider->setValue(lyxrc.currentZoom);