add FILE_STORAGE to Voicemail Build Options

Voicemail can only use one storage module at the moment.
Because it's unclear that selecting one of the storage modules
in menuselect will disable filesystem storage we now have
a FILE_STORAGE option that conflicts with the other modules.

(closes issue #15333)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@200943 65c4cc65-6c06-0410-ace0-fbb531ad65f3