Fix bootstrapped build of QCoreApplication

696060134d introduced the use of
QBasicMutex and QMutexLocker (qmutex.h) but that wasn't #included. I
don't know in what way my build is different, though.

Change-Id: Ie3df3c746fdf1c4735f298c3578cd93a9a14327e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -54,6 +54,7 @@
 #include <qfile.h>
 #include <qfileinfo.h>
 #include <qhash.h>
+#include <qmutex.h>
 #include <private/qprocess_p.h>
 #include <qstandardpaths.h>
 #include <qtextcodec.h>