Clean up copy_file_range() return type

The return type is ssize_t, not int.  Fix that in one instance; the
others were already okay.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/f9aab072-0078-49e4-ab93-3b08086a4406@eisentraut.org

src/bin/pg_combinebackup/reconstruct.c

@@ -695,7 +695,7 @@ write_reconstructed_file(const char *input_filename,
 			 */
 			do
 			{
-				int			wb;
+				ssize_t		wb;
 
 				wb = copy_file_range(s->fd, &off, wfd, NULL, BLCKSZ - nwritten, 0);