tests: Fix fmt-t.c warning

'main' in fmt-t.c was declared as a prototype-less function, which
triggers a warning, which is an error with --enable-checking. Fix it
by declaring 'main' properly.

Change-Id: I45cfec591acd0ef8d7836c79e997e8ffe29b9e38
Reviewed-on: http://gerrit.openafs.org/11539
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Nathaniel Filardo <nwfilardo@gmail.com>