Add the missing include guard

Pick-to: 6.5
Task-number: QTBUG-109394
Change-Id: I6a46784fa65aecd48a7f59021c4942ed52321742
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/plugins/platforms/wasm/qwasmopenglcontext.h

@@ -1,6 +1,9 @@
 // Copyright (C) 2018 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
 
+#ifndef QWASMOPENGLCONTEXT_H
+#define QWASMOPENGLCONTEXT_H
+
 #include <qpa/qplatformopenglcontext.h>
 
 #include <emscripten.h>
@@ -48,3 +51,4 @@ private:
 
 QT_END_NAMESPACE
 
+#endif // QWASMOPENGLCONTEXT_H