Remove the last remaining non-SSL use of QMutexPool in Qt

Task-number: QTBUG-59164
Change-Id: Idd5ceba1eba34cb78c46fffd14a734735991f4fb
Reviewed-by: David Faure <david.faure@kdab.com>

src/network/kernel/qauthenticator.cpp

@@ -1445,7 +1445,8 @@ static PSecurityFunctionTable pSecurityFunctionTable = NULL;
 
 static bool q_NTLM_SSPI_library_load()
 {
-    QMutexLocker locker(QMutexPool::globalInstanceGet((void *)&pSecurityFunctionTable));
+    static QBasicMutex mutex;
+    QMutexLocker l(&mutex);
 
     // Initialize security interface
     if (pSecurityFunctionTable == NULL) {