Add int64 -> int8 mapping to genbki

Per discussion with Tom and Andrew, 64bit integers are no longer a
problem for the catalogs, so go ahead and add the mapping from the C
int64 type to the int8 SQL identification to allow using them.

Patch by Adam Brightwell

src/backend/catalog/Catalog.pm

@@ -33,6 +33,7 @@ sub Catalogs
 	my %RENAME_ATTTYPE = (
 		'int16'         => 'int2',
 		'int32'         => 'int4',
+		'int64'         => 'int8',
 		'Oid'           => 'oid',
 		'NameData'      => 'name',
 		'TransactionId' => 'xid');