qstringfwd.h: add missing QString fwd decl

qstringfwd.h should probably fwd declare QString, too :o

Amends abe3b4c9b9.

Pick-to: 6.5
Change-Id: I80558c92d1144ead0aade410a8b1810713fed701
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit bc4105c843f666eaeaaa44b5a1f897ffa5824ec7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 83e18b6540fafc6f3e0b4700587b026dc08f2d36)

src/corelib/text/qstringfwd.h

@@ -33,6 +33,7 @@ using QLatin1String = QLatin1StringView;
 class QLatin1String;
 using QLatin1StringView = QLatin1String;
 #endif
+class QString;
 class QStringView;
 template <bool> class QBasicUtf8StringView;
 class QAnyStringView;