packet: Avoid array of struct with zero length member.

Windows doesn't like that the Geneve header has an array of
options with each have a zero length member (the variable data).
Nothing is accessing the data now, so just replace the member with
a comment - we can use pointer arithmetic when necessary.

Reported-by: Gurucharan Shetty <[email protected]>
Signed-off-by: Jesse Gross <[email protected]>

lib/packets.h

@@ -730,7 +730,7 @@ struct geneve_opt {
     uint8_t   r2:1;
     uint8_t   r1:1;
 #endif
-    uint8_t   opt_data[];
+    /* Option data */
 };
 
 struct genevehdr {