Fix build with MSVC2019

The error was:

src\virtualkeyboard\qvirtualkeyboardinputcontext_p.cpp(305): error C2678:
binary '!=': no operator found which takes a left-hand operand of type
'QList<QInputMethodEvent::Attribute>' (or there is no acceptable conversion)

This patch fixes the issue by manually defining an operator== for
QInputMethodEvent::Attribute.

Change-Id: Idb283cf7b6ff4388a38ea7780c3d5c1c5f77038d
Fixes: QTBUG-85789
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/gui/kernel/qevent.h

@@ -671,6 +671,19 @@ public:
 
     QInputMethodEvent(const QInputMethodEvent &other);
 
+    inline friend bool operator==(const QInputMethodEvent::Attribute &lhs,
+                                  const QInputMethodEvent::Attribute &rhs)
+    {
+        return lhs.type == rhs.type && lhs.start == rhs.start
+                && lhs.length == rhs.length && lhs.value == rhs.value;
+    }
+
+    inline friend bool operator!=(const QInputMethodEvent::Attribute &lhs,
+                                  const QInputMethodEvent::Attribute &rhs)
+    {
+        return !(lhs == rhs);
+    }
+
 private:
     QString m_preedit;
     QList<Attribute> m_attributes;