Fix compatibility with Qt 5.15.

Patch copied from upstream source repository:

https://bazaar.launchpad.net/~adamreichold/qpdfview/trunk/revision/2104

--- a/sources/model.h	2017-04-19 21:01:25 +0000
+++ b/sources/model.h	2020-06-09 06:24:11 +0000
@@ -24,6 +24,7 @@
 #define DOCUMENTMODEL_H
 
 #include <QList>
+#include <QPainterPath>
 #include <QtPlugin>
 #include <QWidget>
 #include <QVector>

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<tr><td><span title='2023-03-02 10:55:08 +0000'>2023-03-02</span></td><td><a href='/guix/commit/gnu/packages/jemalloc.scm?id=7df09ee0ab3e7962ef27859ce87e06a323059284'>Merge remote-tracking branch 'savannah/master' into core-updates</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2023-02-19 01:00:01 +0100'>2023-02-19</span></td><td><a href='/guix/commit/gnu/packages/jemalloc.scm?id=1e6ddceb8318d413745ca1c9d91fde01b1e0364b'>gnu: Use HTTPS package home pages wherever possible.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>