Use __alignof__ instead of sizeof for stringfield length storage.

Kevin P Fleming suggested that r343157 should use __alignof__ instead
of sizeof. For most systems this won't be an issue, but better fix it
now while it's still fresh.

Review: https://reviewboard.asterisk.org/r/1573
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Merged revisions 344843 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 344845 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@344846 65c4cc65-6c06-0410-ace0-fbb531ad65f3