Add DNGN_EXIT_HELL to is_travelable_stair(), since it already includes

DNGN_ENTER_HELL.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6206 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on June 29, 2008
OOCUMD7ENLTBX2ER7WMZBYWHJ2SHEMP5VLPCRROZMHL33OUWK6VQC
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