Compile virtio-net.c into virtio-net.o during build process

Added compilation rule to include virtio-net.c and generate
virtio-net.o object file in the Makefile. This change ensures
that the virtio-net functionality is compiled and linked correctly
into the project.

Author: jimmylu890303

Makefile

@@ -30,6 +30,7 @@ OBJS := \
 	virtio-pci.o \
 	virtq.o \
 	virtio-blk.o \
+	virtio-net.o \
 	diskimg.o \
 	main.o