[intel] Avoid spurious compiler warning on GCC 10

GCC 10 produces a spurious warning about an out-of-bounds array access
for the unsized raw dword array in union intelvf_msg.

Avoid the warning by embedding the zero-length array within a struct.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/intelvf.h

@@ -119,6 +119,12 @@ struct intelvf_msg_queues {
 	uint32_t dflt;
 } __attribute__ (( packed ));
 
+/** Raw mailbox message */
+struct intelvf_msg_raw {
+	/** Raw dwords */
+	uint32_t dword[0];
+} __attribute__ (( packed ));
+
 /** Mailbox message */
 union intelvf_msg {
 	/** Message header */
@@ -132,7 +138,7 @@ union intelvf_msg {
 	/** Queue configuration message */
 	struct intelvf_msg_queues queues;
 	/** Raw dwords */
-	uint32_t dword[0];
+	struct intelvf_msg_raw raw;
 };
 
 /** Maximum time to wait for mailbox message