app_voicemail: fix missing symbol

ASTERISK-23391 caused a regression where the symbol 'defaultlanguage'
was used by app_voicemail but not exported by main/asterisk.  This
change renames the variable to ast_defaultlanguage.  The variable was
already renamed in Asterisk 12+.

(closes issue ASTERISK-23559)
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3408/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@411633 f38db490-d61c-443f-a65b-d21fe96a405b