Ignore db/structure.sql file in TrailingWhitespace hook

This file is generated by Rails and sometimes contains trailing
whitespace. We don't want to bother complaining about trailing
whitespace in this case.

Change-Id: I71ae2427dcadf076695298f44812433a1681088d
Reviewed-on: http://gerrit.causes.com/40356
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>