Increase the number of pre-allocated hfs (from 144k to 170k).

Increase the number of pre-allocated hfs (from 144k to 170k).

169756 header fields are currently registered.  Including the 5k expert info
field pre-allocation this gives us about 5k unused entries (until we should
increase this number again).

Change-Id: Id8ea979cbe9e80756c12b5b73e816bbe3e9856ad
Reviewed-on: https://code.wireshark.org/review/10599
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>