Document Third-Party Code changes in Qt 5.12.3

The changes where obtained by running

qtattributionsscanner -output-format json

in the sources of both Qt 5.12.2 and Qt 5.12.3 checkouts, and diff'ing
these.

Fixes: QTBUG-75026
Change-Id: If1b8b44c45333d33e8b163639c6d6a0e81798d53
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>

doc/src/legal/licensechanges.qdoc

@@ -34,6 +34,13 @@
     Changes in Qt and \l{Licenses Used in Qt}{Third Party Modules}
     released with Qt that are relevant to licensing.
 
+    \section1 Qt 5.12.3
+
+    \section2 Qt GUI Module
+
+    The license of \l{Wintab API} was now classified as 'Custom' instead of
+    'Public Domain'.
+
     \section1 Qt 5.12.2
 
     \section2 Qt GUI Module