checkkconfigsymbols.py: Fix the '--ignore' option

It seems like the implementation of the --ignore option is broken.

In check_symbols_helper, when going through the list of files, a file is
added to the list of source files to check if it matches the ignore
pattern. Instead, as stated in the comment below this condition, the
file should be added if it doesn't match the pattern.

This means that when providing an ignore pattern, the only files that
will be checked will be the ones we want the ignore, in addition to the
Kconfig files that don't match the pattern (the check in
parse_kconfig_files is done right)

Signed-off-by: Ariel Marcovitch <arielmarcovitch@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>