avio: cosmetics, prettify AVIO_SEEKABLE_NORMAL

Move the doxy above the definition, change the value itself to the
(1 << n) pattern, which is more readable for flags.

libavformat/avio.h

@@ -34,8 +34,10 @@
 
 #include "libavformat/version.h"
 
-
-#define AVIO_SEEKABLE_NORMAL 0x0001 /**< Seeking works like for a local file */
+/**
+ * Seeking works like for a local file.
+ */
+#define AVIO_SEEKABLE_NORMAL (1 << 0)
 
 /**
  * Callback for checking whether to abort blocking functions.