libstdc++ is required for the qga-vss.dll that provides fsfreeze
functionality. Currently it is not provided by the MSI installer,
resulting in fsfreeze being disabled in guest environments where it has
not been installed by other means.

In the future this would be better handled via gcc-cpp ComponentGroup
provided by msitools, but that would be better handled with a general
rework of DLL dependency handling in the installer build. Keep it
simple for now to fix this regression.

Tested with Fedora 34 mingw build environment.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Kostiantyn Kostiuk <konstantin@daynix.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>