Fix build with newer GCC.

Taken from upstream:

  https://sourceforge.net/p/libwpd/code/ci/333c8a26f231bea26ec3d56245315041bbf5577f/

--- a/src/lib/WPXTable.h
+++ b/src/lib/WPXTable.h
@@ -53,7 +53,7 @@
 	~WPXTable();
 	void insertRow();
 	void insertCell(unsigned char colSpan, unsigned char rowSpan, unsigned char borderBits);
-	const WPXTableCell  *getCell(size_t i, size_t j)
+	const WPXTableCell  *getCell(std::size_t i, std::size_t j)
 	{
 		return &(m_tableRows[i])[j];
 	}
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<tr><td><span title='2020-10-07 13:54:50 +0200'>2020-10-07</span></td><td><a href='/guix/commit/gnu/packages/nano.scm?id=e6b2a3eba5fc0720046ffdd9458dd83c89c4b85f'>gnu: nano: Update to 5.3.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nano.scm (nano): Update to 5.3.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-08-24 13:28:22 +0200'>2020-08-24</span></td><td><a href='/guix/commit/gnu/packages/nano.scm?id=5abbf435fce8c2245175dcb9b62e5722cfaadc26'>gnu: nano: Update to 5.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nano.scm (nano): Update to 5.2.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-08-12 12:45:28 +0200'>2020-08-12</span></td><td><a href='/guix/commit/gnu/packages/nano.scm?id=5b893f43982f95e8d4d5fdf0ec3b9cf16017fb4e'>gnu: nano: Update to 5.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nano.scm (nano): Update to 5.1.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-07-29 13:07:24 +0200'>2020-07-29</span></td><td><a href='/guix/commit/gnu/packages/nano.scm?id=c09f22aaa68e8ee02beee0e91ffc87cbafe3e2f8'>gnu: nano: Update to 5.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/nano.scm (nano): Update to 5.0.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>