Update libnghttp2 to latest master (add support for draft-13)

Bug:10256
Change-Id: I24275f1b67120f69cfd673f7e5598a50b3c7566f
Reviewed-on: https://code.wireshark.org/review/2145
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>