devtools: fix directory filter in forbidden token check

checkpatches.sh current complains on a patch [1] adding
ALLOW_EXPERIMENTAL_API in an example while this check is for app, lib
and drivers directories:

Warning in examples/ethtool/ethtool-app/Makefile:
Using experimental build flag for in-tree compilation

The regexp on entering files concerned by this filter is incorrect.
In the [1] case, the file full name is matched against "app" rather than
"+++ b/app".

1: https://patchwork.dpdk.org/patch/83902/

Fixes: 7413e7f2ae ("devtools: alert on new calls to exit from libs")
Cc: stable@dpdk.org

Signed-off-by: David Marchand <david.marchand@redhat.com>

devtools/check-forbidden-tokens.awk

@@ -54,7 +54,7 @@ BEGIN {
 	}
 	for (i in deny_folders) {
 		re = "^\\+\\+\\+ b/" deny_folders[i];
-		if ($0 ~ deny_folders[i]) {
+		if ($0 ~ re) {
 			in_file = 1
 			last_file = $0
 		}