qtqa: Set qt_update_submodules.pl to use qtx11extras' master branch

QtX11Extras is missing dev/stable/release brances, so Qt submodule
update bot is usign only its master branch.

Change-Id: I7400864cd7bb66543604e8b339d56a6e65862f9b
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: David Faure (KDE) <faure@kde.org>

scripts/qt/qt_update_submodules.pl

@@ -87,6 +87,7 @@ Readonly my @PROPERTIES => (
 Readonly my %SUBMODULE_TRACKING_REF => (
     qtrepotools  =>  'refs/heads/master',
     qtqa         =>  'refs/heads/master',
+    qtx11extras  =>  'refs/heads/master',
 );
 
 # Author and committer to be used for commits by this script.