[LyX/master] Add missing check for Qt version when using QPdfWriter class

[Stephan Witt]

commit 0273d607c50b41a7732523631d571e3a28fa2a02
Author: Stephan Witt <sw...@lyx.org>
Date:   Tue Oct 3 22:49:23 2017 +0200

    Add missing check for Qt version when using QPdfWriter class
---
 src/convert/lyxconvert.cpp |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/src/convert/lyxconvert.cpp b/src/convert/lyxconvert.cpp
index 38ca49f..27f361f 100644
--- a/src/convert/lyxconvert.cpp
+++ b/src/convert/lyxconvert.cpp
@@ -8,7 +8,9 @@ g++ -std=gnu++11 $cflags lyxconvert.cpp -o lyxconvert $libs
 #include <QImage>
 #include <QFile>
 #include <QPainter>
+#if (QT_VERSION >= 0x050300)
 #include <QPdfWriter>
+#endif
 
 
 const char * basename(const char * name)