qobject_p.h: remove unused qreadwritelock.h include

Only qcoreapplication_p.h uses a QReadWriteLock (tanslationMutex), so
include it only there.

This include has caused QAtomicPointer<QReadWriteLockPrivate> to
percolate to the top-5 of template instantiation hogs when compiling
QtWidgets, as reported by Clang -ftime-trace.

This patch improves that, because qcoreapplication_p.h isn't part of
the PCH and is much less frequently included into QtWidgets TUs (68
instead of 142 times).

Pick-to: 6.3 6.2
Change-Id: I2518040d83a04e7cef0645d7f4bf641fb50f49ff
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qcoreapplication_p.h

@@ -19,6 +19,7 @@
 #if QT_CONFIG(commandlineparser)
 #include "QtCore/qcommandlineoption.h"
 #endif
+#include "QtCore/qreadwritelock.h"
 #include "QtCore/qtranslator.h"
 #if QT_CONFIG(settings)
 #include "QtCore/qsettings.h"

src/corelib/kernel/qobject_p.h

@@ -21,7 +21,6 @@
 #include "QtCore/qlist.h"
 #include "QtCore/qobject.h"
 #include "QtCore/qpointer.h"
-#include "QtCore/qreadwritelock.h"
 #include "QtCore/qsharedpointer.h"
 #include "QtCore/qvariant.h"
 #include "QtCore/qproperty.h"