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>