[testsuite] Fix dg-extract-results.sh path

There was a typo in patch:
commit 5a699617
Author: Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE>
Date:   Mon Aug 6 16:05:16 2018 +0200
    Update dg-extract-results.* from gcc

gdb/testsuite/ChangeLog
2018-09-11  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* Makefile.in (check-parallel-racy): Fix dg-extract-results.sh path.