arm64: Check for virtio for scmi_virtio.c

scmi_virtio.c depends on virtio. Check for this before including it in
the kernel.

Reported by:	Isaac Cilia Attard (via cperciva)
Sponsored by:	Arm Ltd

Author: zxombie

sys/conf/files.arm64

@@ -275,7 +275,7 @@ dev/firmware/arm/scmi_clk.c			optional fdt scmi
 dev/firmware/arm/scmi_if.m			optional fdt scmi
 dev/firmware/arm/scmi_mailbox.c			optional fdt scmi
 dev/firmware/arm/scmi_smc.c			optional fdt scmi
-dev/firmware/arm/scmi_virtio.c			optional fdt scmi
+dev/firmware/arm/scmi_virtio.c			optional fdt scmi virtio
 dev/firmware/arm/scmi_shmem.c			optional fdt scmi
 
 dev/gpio/pl061.c				optional pl061 gpio