avio: Fix the value of the deprecated URL_FLAG_NONBLOCK

This isn't used in practice anywhere within libav at the moment,
but change it for consistency until it is removed.
URL_RDONLY/WRONLY were fixed in commit 5b81e295 (after the
values that actually were used were changed at the major bump,
in commit cbea3ac8), but this flag was unintentionally left unfixed.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/avio.h

@@ -204,7 +204,7 @@ attribute_deprecated int url_poll(URLPollEntry *poll_table, int n, int timeout);
  * Warning: non-blocking protocols is work-in-progress; this flag may be
  * silently ignored.
  */
-#define URL_FLAG_NONBLOCK 4
+#define URL_FLAG_NONBLOCK 8
 
 typedef int URLInterruptCB(void);
 extern URLInterruptCB *url_interrupt_cb;