QSemaphore: drop unneeded unnamed namespace around QtSemaphorePrivate

Fixes GCC 11 C++17 unity-build (didn't investigate which one is the
problem) error:

    qsemaphore.cpp:275:7: error: ‘QSemaphorePrivate’ has a base ‘{anonymous}::QtSemaphorePrivate::Layout1’ whose type uses the anonymous namespace [-Werror=subobject-linkage]
      275 | class QSemaphorePrivate : public QtSemaphorePrivate::Members
          |       ^~~~~~~~~~~~~~~~~

Amends fbc491230f.

Change-Id: I3b95a35822ec62aabd4c310b02bf58679bb9651f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/thread/qsemaphore.cpp

@@ -251,7 +251,7 @@ futexSemaphoreTryAcquire(QBasicAtomicInteger<quintptr> &u, int n, T timeout)
     return false;
 }
 
-namespace { namespace QtSemaphorePrivate {
+namespace QtSemaphorePrivate {
 using namespace QtPrivate;
 struct Layout1
 {
@@ -270,7 +270,7 @@ struct Layout2
 // Choose Layout1 if it is smaller than Layout2. That happens for platforms
 // where sizeof(mutex) is 64.
 using Members = std::conditional_t<sizeof(Layout1) <= sizeof(Layout2), Layout1, Layout2>;
-} } // namespace QtSemaphorePrivate
+} // namespace QtSemaphorePrivate
 
 class QSemaphorePrivate : public QtSemaphorePrivate::Members
 {