LU-12923 contrib: Update spelling.txt to add BUILD_BUG_ON()

LU-12923 contrib: Update spelling.txt to add BUILD_BUG_ON()

This is first in the series of patchs which replaces
CLASSERT() with upstream kernel defined BUILD_BUG_ON()

This specific patch updates contrib/scripts/spelling.txt
to add line CLASSERT||BUILD_BUG_ON(). This will subsequently
help follow up patchs to trap and flag warning during
checkpatch check if CLASSERT() is still left defined.

Test-Parameters: trivial
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: If8fd76dd107cb53d657b7fa89bd62a9357222629
Reviewed-on: https://review.whamcloud.com/36645
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Neil Brown <neilb@suse.de>