QString: avoid a QLocale detach in vsprintf

I didn't dig deeper to check where the additional reference came from,
but tracing confirmed that the QSharedDataPointer<QLocalePrivate>::detach()
call is gone with this change.

Background:
QString::vsprintf is used in the logging framework, and as such shouldn't
waste any memory. It's currently anyway unusable from, say, signal handlers,
but this is low-hanging fruit that shaves off one of the dozen or so
memory allocations involved in a simple qFatal() call.

Change-Id: I19fa2148f669dfc7b5f276221151e25a4348cbfe
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/corelib/tools/qstring.cpp

@@ -5505,7 +5505,7 @@ QString &QString::sprintf(const char *cformat, ...)
 
 QString &QString::vsprintf(const char* cformat, va_list ap)
 {
-    QLocale locale(QLocale::C);
+    const QLocale locale(QLocale::C);
 
     if (!cformat || !*cformat) {
         // Qt 1.x compat