billing: Add shebang header to indexer cronjob

Addresses an issue with the billing indexer cronjob, that
prevented it from being executed.

Target: trunk
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6249/
(cherry picked from commit d73114fca0cf47942b68a275de296e3b6d5fc3c3)