Fix searchpath for modern Perl for genbki.pl

This was fixed for MSVC tools by commit 1df92eeafefac4, but per
buildfarm member bowerbird genbki.pl needs the same treatment.

Backpatch to all live branches.

(cherry picked from commit 51884fa16a179852ec851172a72588d8e84ec05e)

src/backend/catalog/genbki.pl

@@ -14,6 +14,10 @@
 #
 #----------------------------------------------------------------------
 
+use File::Basename;
+use File::Spec;
+BEGIN  { use lib File::Spec->rel2abs(dirname(__FILE__)); }
+
 use Catalog;
 
 use strict;