Ahaggar: fix a bug that prevented us from showing the correct location

Fix a bug that prevented us from showing the correct location of errors
and warnings.

Use the strrchr() to search for [location] instead of strchr() since the
location is always reported at the end by the ahaggar-fncalls plugins.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>