QDockWidget: Fix icons being too large when using a style proxying Windows style

Extend the check for Windows style to QProxyStyle. Amends
43b9c9b993.

Task-number: QTBUG-38776
Change-Id: I5331d0c29bea59c59309feeab153437a72e0e30f
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/widgets/qdockwidget.cpp

@@ -45,6 +45,7 @@
 #include <qdrawutil.h>
 #include <qevent.h>
 #include <qfontmetrics.h>
+#include <qproxystyle.h>
 #include <qwindow.h>
 #include <qscreen.h>
 #include <qmainwindow.h>
@@ -166,6 +167,10 @@ static inline bool isWindowsStyle(const QStyle *style)
     if (style->inherits("QStyleSheetStyle"))
       effectiveStyle = static_cast<const QStyleSheetStyle *>(style)->baseStyle();
 #endif
+#if !defined(QT_NO_STYLE_PROXY)
+    if (style->inherits("QProxyStyle"))
+      effectiveStyle = static_cast<const QProxyStyle *>(style)->baseStyle();
+#endif
 
     return effectiveStyle->inherits("QWindowsStyle");
 }