Ignore .exe files in whitespace test

Ignore .exe files in whitespace test

On windows, we sometimes require getopt executables, which end up
in the source tree. These shouldn't break the whitespace test.

Change-Id: Iaf86e38b94605bebb69a317e00f932eefcf468b9
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/863
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>