Makefile: Only build virtiofsd if system-mode is enabled

Makefile: Only build virtiofsd if system-mode is enabled

Do not build the virtiofsd helper when configured with
--disable-system.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200522172510.25784-2-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>