Correct copyright for newly added genericdesc.c

It's 2016 these days (no, not entirely sure how we got here either).

Pointed out by Amit Langote

src/backend/access/rmgrdesc/genericdesc.c

@@ -4,7 +4,7 @@
  *	  rmgr descriptor routines for access/transam/generic_xlog.c
  *
  *
- * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * src/backend/access/rmgrdesc/genericdesc.c