Fix enum definition.

Commit 249cf070 assigned to one of
the labels in the middle the value that should have been assigned
to the first member of the enum.  Rushabh's patch didn't have that
defect as submitted, but I managed to mess it up while editing.
Repair.

src/include/pgstat.h

@@ -816,7 +816,7 @@ typedef enum
  */
 typedef enum
 {
-	WAIT_EVENT_BUFFILE_READ,
+	WAIT_EVENT_BUFFILE_READ = PG_WAIT_IO,
 	WAIT_EVENT_BUFFILE_WRITE,
 	WAIT_EVENT_CONTROL_FILE_READ,
 	WAIT_EVENT_CONTROL_FILE_SYNC,
@@ -829,7 +829,7 @@ typedef enum
 	WAIT_EVENT_DATA_FILE_FLUSH,
 	WAIT_EVENT_DATA_FILE_IMMEDIATE_SYNC,
 	WAIT_EVENT_DATA_FILE_PREFETCH,
-	WAIT_EVENT_DATA_FILE_READ = PG_WAIT_IO,
+	WAIT_EVENT_DATA_FILE_READ,
 	WAIT_EVENT_DATA_FILE_SYNC,
 	WAIT_EVENT_DATA_FILE_TRUNCATE,
 	WAIT_EVENT_DATA_FILE_WRITE,