Correct Copyright year from 2015 to 2016

Looks like this patch went in after Copyright messages
were updated for 2016 and it missed the boat. Fixed.

src/include/storage/standbydefs.h

@@ -4,7 +4,7 @@
  *	   Frontend exposed definitions for hot standby mode.
  *
  *
- * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * src/include/storage/standbydefs.h