scripts/headers_check.pl: correct RE in header CONFIG leak check

Correct the regular expression in scripts/headers_check.pl to include '_'
as a valid character in the class; otherwise, the check will report a
"leaked" symbol of CONFIG_A_B_C as merely CONFIG_A.

This patch will make no difference whatsoever in the current kernel tree
as the call to the perl routine that does that check is currently
commented out:

                &check_include();
                &check_asm_types();
                &check_sizetypes();
                &check_prototypes();
                # Dropped for now. Too much noise &check_config();

However, I noticed that problem when I was building the yum downloadable
kernel source rpm for fedora 11 (beta), which *does* run that check, and
that's where the problem became obvious.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>