There is no actual code in the CONFIG_VIRGL=n case.  So building is
(a) pointless and (b) makes macos ranlib complain.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20201026142851.28735-1-kraxel@redhat.com