windeployqt: Fix typo in mkspec detection

When adding support for ARM64 hosts a typo sneaked into the mkspec
detection. This typo broke deployment for mingw builds.

This patch amends 81984807b3

Pick-to: 6.7
Fixes: QTBUG-126775
Change-Id: I37ee3617dc6b92520250da3ff0665ce06cea1cb8
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Wladimir Leuschner <wladimir.leuschner@qt.io>
(cherry picked from commit 3b789ed01e1efead3538b7cdc08f456e55f1d882)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/tools/windeployqt/main.cpp

@@ -139,7 +139,7 @@ static Platform platformFromMkSpec(const QString &xSpec)
             return WindowsDesktopClangMsvc;
         if (xSpec.contains("arm"_L1))
             return WindowsDesktopMsvcArm;
-        if (xSpec.contains("G++"_L1))
+        if (xSpec.contains("g++"_L1))
             return WindowsDesktopMinGW;
 
         return WindowsDesktopMsvc;