libnghttp2: Update to 0.6.4

Change-Id: Ic8878f818a89ebc1056777a5b0838e8a5b1215d5
Reviewed-on: https://code.wireshark.org/review/4834
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>