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.

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;