Fix build with qt-namespace

Add missing include of global module header.
Drop unneeded global namespace qualifier for constant in
our namespace.

Amends 6017695bfa

Change-Id: I04633030af13df3c3359fc09b08b4a5f031013a9
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/platform/windows/qwindowsthemecache.cpp

@@ -22,7 +22,7 @@ Q_GLOBAL_STATIC(ThemesCache, themesCache);
 QString QWindowsThemeCache::themeName(int theme)
 {
     return theme >= 0 && theme < int(std::size(themeNames))
-        ? QString::fromWCharArray(::themeNames[theme]) : QString();
+        ? QString::fromWCharArray(themeNames[theme]) : QString();
 }
 
 HTHEME QWindowsThemeCache::createTheme(int theme, HWND hwnd)

src/gui/platform/windows/qwindowsthemecache_p.h

@@ -15,6 +15,8 @@
 // We mean it.
 //
 
+#include "QtGui/private/qtguiglobal_p.h"
+
 #include <QtCore/qt_windows.h>
 #include <uxtheme.h>