tool_main.c: remove redundant vms_show storage-class specifier

vms_show 'extern' storage-class specifier removed from tool_main.c due to...

- Advice from Tor Arntsen: http://curl.haxx.se/mail/lib-2013-03/0164.html

- HP OpenVMS docs stating that 'Extern is the default storage class for
  variables declared outside a function.'
  http://h71000.www7.hp.com/commercial/c/docs/dec_c_help_5.html
  (Storage_Classes section)