pgrowlocks relkind check

The following commit added relkind checks to certain contrib modules so
that a more user-friendly error is produced if the wrong kind of relation
is passed to its functions:

commit c08d82f38ebf763b79bd43ae34b7310ee47aaacd
Author: Stephen Frost <sfrost(at)snowman(dot)net>
Date: Thu Mar 9 16:34:25 2017 -0500

Add relkind checks to certain contrib modules

But it missed pgrowlocks, so the following happens:

create extension pgrowlocks;
create view one as select 1;
select pgrowlocks('one');
-- ERROR: could not open file "base/68730/68748": No such file or directory

With the attached patch:

select pgrowlocks('one');
ERROR: "one" is not a table, index, materialized view, sequence, or TOAST
table

Thanks,
Amit