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='2021-06-06 19:49:57 +0200'>2021-06-06</span></td><td><a href='/guix/commit/gnu/packages/patches/nss-increase-test-timeout.patch?id=79878c64b4bda523f50974c3f61b796309b1b8d9'>gnu: nss: Update to 3.66.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2021-01-26 14:22:34 -0500'>2021-01-26</span></td><td><a href='/guix/commit/gnu/packages/patches/nss-increase-test-timeout.patch?id=ced3d5cbf99eac429a45e33191335ec0662f1df3'>gnu: nss: Update to the latest changeset to resolve test failures.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>