Fix whitespace lint to ignore binary files

This lint would sometimes incorrectly report the existence of a hard tab
within a binary file. The solution was to add the -I flag to the grep
call to explicitly ignore all binary files.

Change-Id: Idf121a4de6c19b589b80a853b5aec6a5f89a9af2
Reviewed-on: https://gerrit.causes.com/21912
Reviewed-by: John Skopis <john.skopis@causes.com>
Tested-by: Shane da Silva <shane@causes.com>