avoid double typedef

[Daniel Kahn Gillmor]

On IRC, Domo pointed out that older gcc complains when a typedef gets
repeated.  So i'm updating patch 8 in this series to avoid double
typedefs.  The rest of the series should be unchanged, so i'm avoiding
re-flooding the lst with them, but i'm happy to send along a full
round of v4 if folks would prefer that.

the "cleartext-index" branch of https://gitlab.com/dkg/notmuch has
been updated to include this fix, and is now at
9be9d7a6bf083968458ad2cecf4af7f0d99f1cb3

Regards,

        --dkg