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>

://git.koszko.org/cgit-static/cgit.png' alt='cgit logo'/></a></td>
<td class='main'><a href='/'>index</a> : <a href='/guix/'>guix</a></td><td class='form'><form method='get'>
<input type='hidden' name='id' value='1ad332b367347bbd91f8a4527e271945f2c45ae6'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=1ad332b367347bbd91f8a4527e271945f2c45ae6'>refs</a><a class='active' href='/guix/log/bootstrap'>log</a><a href='/guix/tree/bootstrap?id=1ad332b367347bbd91f8a4527e271945f2c45ae6'>tree</a><a href='/guix/commit/bootstrap?id=1ad332b367347bbd91f8a4527e271945f2c45ae6'>commit</a><a href='/guix/diff/bootstrap?id=1ad332b367347bbd91f8a4527e271945f2c45ae6'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/bootstrap'>
<input type='hidden' name='id' value='1ad332b367347bbd91f8a4527e271945f2c45ae6'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=1ad332b367347bbd91f8a4527e271945f2c45ae6'>root</a>/<a href='/guix/log/bootstrap?id=1ad332b367347bbd91f8a4527e271945f2c45ae6'>bootstrap</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/bootstrap?id=1ad332b367347bbd91f8a4527e271945f2c45ae6&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-02-17 11:49:08 +0100'>2020-02-17</span></td><td><a href='/guix/commit/bootstrap?id=3dd311e3a059131ef245417106d4fb659222ef3c'>bootstrap: Fix typo.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-09-18 11:38:14 +0200'>2019-09-18</span></td><td><a href='/guix/commit/bootstrap?id=7bc46ecc34ceb042ff890e5d306d0a2a736e891d'>doc: Add Guix Cookbook.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2019-04-26 11:21:32 +0200'>2019-04-26</span></td><td><a href='/guix/commit/bootstrap?id=9ca5ff882e2ac4eaab02eb0fde545bd784af478b'>bootstrap: Break automake dependency on generated files.</a><span class='msg-avail'>...</span></td><td>Miguel Ángel Arruga Vivas</td></tr>