devtools: fix directory filter in forbidden token check

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: 7413e7f2aeb3 ("devtools: alert on new calls to exit from libs")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>