Add "contrib" to the list of GDB support dirs (on src-release.sh)

Add "contrib" to the list of GDB support dirs (on src-release.sh)

On commit:

  commit 5a6996172e6294ea37054b1a9caa3a923a8fe399
  Author: Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE>
  Date:   Mon Aug 6 16:05:16 2018 +0200

      Update dg-extract-results.* from gcc

dg-extract-results.sh was moved from the "gdb/contrib/" directory to
the toplevel "contrib/" directory.  However, src-release.sh was not
updated in order to include "contrib/" in the tarball release of GDB.
This makes it very inconvenient to run and analyze the GDB testsuite
results.  This commit adds "contrib/" to the list of support
directories that are included in each GDB release.

ChangeLog:
2018-09-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	* src-release.sh (GDB_SUPPORT_DIRS): Add "contrib".