Have postgres.bki self-identify

Hi!

While trying to find postgres.bki in my build directory searching for the
file name didn't work because there is no comment in the file containing
the file name; like there is in every other file we write or generate,
including the related *_d.h files. Add it.

It probably belongs before the version identifier but wasn't sure if that
placement would be considered something worth retaining.

diff --git a/src/backend/catalog/genbki.pl b/src/backend/catalog/genbki.pl
index 2501307c92..175fde4acc 100644
--- a/src/backend/catalog/genbki.pl
+++ b/src/backend/catalog/genbki.pl
@@ -458,6 +458,9 @@ open my $syscache_info_fh, '>', $syscache_info_file .
$tmpext
# version marker for .bki file
print $bki "# PostgreSQL $major_version\n";

+# self-identify file in comment per code policies.
+print $bki "# postgres.bki\n";
+
# vars to hold data needed for schemapg.h
my %schemapg_entries;
my @tables_needing_macros;

David J.