Fix QBasicMutex default constructor not constexpr

Commit 02dc39fa8e added the constructor
for the bootstrapped mode. For the regular mode, we hadn't needed, since
the {} syntax guaranteed initialization for us.

Turns out there's at least one compiler that doesn't think it was enough
(GCC for QNX 7).

Task-number: QTBUG-64451
Change-Id: Ic632b4163d784b83951cfffd14f6766b4cb4eb64
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/corelib/thread/qmutex.h

@@ -67,6 +67,12 @@ class QMutexData;
 class Q_CORE_EXPORT QBasicMutex
 {
 public:
+#ifdef Q_COMPILER_CONSTEXPR
+    constexpr QBasicMutex()
+        : d_ptr(nullptr)
+    {}
+#endif
+
     // BasicLockable concept
     inline void lock() QT_MUTEX_LOCK_NOEXCEPT {
         if (!fastTryLock())