Bump WAL version.

Oversight in commit e6d80695. Since that commit changed the format of
XLOG_DBASE_DROP WAL record, XLOG_PAGE_MAGIC needs to be bumped.

Spotted by Michael Paquier

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD101	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD102	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {