Allow const QemuOptDesc

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/net.c b/net.c
index 88934df..a1bf49f 100644
--- a/net.c
+++ b/net.c
@@ -840,7 +840,7 @@
 /* magic number, but compiler will warn if too small */
 #define NET_MAX_DESC 20
 
-static struct {
+static const struct {
     const char *type;
     net_client_init_func init;
     QemuOptDesc desc[NET_MAX_DESC];