devel/libevent2: update to 2.1.11
Changes: https://github.com/libevent/libevent/releases/tag/release-2.1.11-stable
ABI: https://abi-laboratory.pro/tracker/timeline/libevent/
PR: 239599
Reported by: GitHub (watch releases)
Approved by: zeising (maintainer)
MFH: 2019Q3 (maybe security, partially restores 2.1.8 ABI)
Differential Revision: https://reviews.freebsd.org/D21133