`make installer` with a DLL directory was broken.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20201117190640.390359-1-sw@weilnetz.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>