checkkconfigsymbols.py: add option -i to ignore files

Sometimes a user might be interested to filter certain reports (e.g.,
the many defconfigs).  Now, this can be achieved by specifying a Python
regex with -i / --ignore.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>